Just NO.
Haven't read the thread... don't need to. The title says it all.
At that point its not called swimming, its called SINKING.
Put enough SP into anything and you can succeed. Rolling a 20 doesn't hurt either.
If you are carrying less than 16 lbs, you must make a swim check to move at 1/4 speed. If you are carrying more than 16 lbs, you can make a swim check to move 1/4 speed or you can walk on the bottom at 2x cost (10' per 5' square) as difficult terrain (4x if it actually is difficult terrain).
You know, the problem with trying to make a game realistic, is that it's an impossible goal. And trying to pursue that, make other easier goals to fail. As Patton once said, the main enemy of a good plan, is the idea of a perfect plan.
With your rule, a character carrying 16 pounds of wood, would sink. That makes embracing a piece of a mast worthless in a shipwreck.
weight is meaningless, in order to sink. The thing that matters is density. I could jump into the sea with a ton of cork, and that would not make me drown.
Don't know about you, but I'm not thrilling with the idea of the adventurer's armory listing the density of every piece of gear you carry in your bag, and having to calculate the average of it to see if the weight carried is either a hindrance or a benefit to see if the player sinks.
I am fine with a Fighter in Heavy Plate happily wrestling with a sharkopus in stormy seas after swimming 2-3 hours with no chance of drowning, ever.
IF HE HAS PUT RANKS IN SWIM. No other checks seem to be so ridiculously easy as swim.
2-3 hours of swimming, character should be fatigued, so -2 to str.
Assuming starting Str of 18, now down to 16, so +3 to str based skills.1 rank + class skill + str bonus put him at 7.
Swimming in stormy seas, DC 20.
This guy is not going to survive long. Sure he'll make some checks, but after a few consecutive failed checks he is going to be so far underwater he is not coming back up.
Boost him to 4 ranks for a net skill of 10 and he has a 50/50 on every check. His chances are still pretty grim. Consecutive successful checks do not make his situation better (he is either above the water or he is not). Consecutive failed checks do make his situation worse though.
Boost him another 5 ranks (min level required is now 9) and he will very likely survive till he changes from fatigued to exhausted. But he has now put 9 ranks in the skill to have a fairly certain outcome of survival (though still no guarantee with a streak of bad rolls).
So you are complaining about putting 1 rank in the swim skill will allow a character to survive in an extreme condition, yet when you actually look at the numbers it would take an extreme amount of good fortune for him to be able to do so. He will probably survive a few rounds, but after that he is holding his breath constantly and will be dead soon. Or in other words, you are complaining about the unrealistic nature of something that doesn't actually play out using the game's mechanics.
I think you missed the part of the discussion pointing out that failing Swim checks has no correlation to you sinking any distance.
There is simply no rules for sinking, much less a sinking speed per round/failed check.
If there were such a rule, then failing Swim checks several rounds in a row would be much more meaningful, especially in deep water,
as the number of failed checks would correlate to how deep you sink, and thus how many succesful check you need to reach the surface again to catch your breath. But nothing like that is suggested by RAW.
I think such a rule, with or without doubled ACP if desired, would make the Swim rules much more coherent.
As pointed out, sinking speed has to do with buoyancy not weight per se, but as a game, this doesn't need to be exact,
a rule of thumb could correlate weight to object/creature size and relate that to sinking speed.
Of course, with enough weight:volume, even when you are swimming you should be sinking.
(and that can be desired in many cases, e.g. diving quickly)
A successful swim check should just negate a certain amount of sinking distance per check (and usage of the allowed move actions from a successful swim check could negate a further amount if your swim movement is dedicated to that).
Fatigue rules do impact things to a certain extent, but unless Fatigue is hindering your ability to pass the check on a Nat 20, and/or the number of rounds it takes to pass a check (statistically, or Taking 20) creates problems for your Holding Breath/CON checks, then you will eventually pass a check one round, catching your breath/being above the surface/being able to move at least that one round.
Whether or not you pass the check or don't (and are thus "Off-Balance") doesn't substantially affect non-Casting actions you may take, other than whether you can move or not in a given round*, which works out to your speed over a longer time period (i.e. if you take 20 rounds to move the same distance it takes somebody else 5 rounds, simply because you weren't moving for 15 rounds even though your speed is technically the same.) If that is in a combat situation, other than Casting you can take the exact same non-movement actions as somebody who passed the check (the only other difference, which I didn't point out before, is that Piercing weapons suffer the same penalties that Slashing/Bludgeoning weapons do on a passed check). Now, not being able to easily move in-combat is itself a serious impediment, especially for non-Casting actions, but that (and the Piercing penalty) seems much weaker of a consequence for e.g. failing 5 Swim checks in a row due to Heavy Armor/Load than most people feel is reasonable. In an out-of-combat situation there isn't usually much time pressure, so if you can pass it on a Nat 20 you will eventually cross a mile-wide river, for example (at most 20x slower, or proportionately faster if you don't actually need a Nat 20).
Again, I think having actual rules for sinking would make failing several Swim checks in a row much more meaningful (presumably failing by 5+ would lead to sinking?) and in fact make the RAW more in line with what many seem to think the actual state of affairs is, i.e. sinking is a threat of failing a Swim check.
* Incidentally, the "Combat Adjustments Underwater" Table in the Environment chapter (why isn't this in the Combat chapter under the Movement sub-heading?) lists a "Failed Check" as also having "Normal Movement"!!! I mean, you can define "normal" as per defined by the Skill = no movement on a failed check, but it's the exact same listing as for Passed Check, Swim Speed, or Freedom of Movement, so listing that in the table is flat-out misleading, confusing, and non-helpful.
** A question about another of the rules that are all split up from each other:
In the "Water Dangers" section of Environment (the very end), it states:
That non-lethal/lethal damage is another hazard which makes failing several checks in a row actual meaningful in a detrimental way. But that only applies to fast-moving water and is associated with DC15, it doesn't even mention the normal categories at all (Rough, Stormy). Should that apply to ANY Rough and possibly also Stormy? Per RAW, if there isn't a fast-moving current, NO. Also per RAW, that affect is only DC15, even if the fast-moving water also qualifies as Stormy(DC20), so per RAW you can avoid that damage if you pass DC15, even if you fail the higher DC20 and thus "go under" and can't move that round. (The way that line is written, it's actually a separate, secondary check, not the same check, even though failing ALSO means you "go under" just like the normal Swim check) None of that is game-breaking per se, but it's just another weird discrepancy lost between the jumble of rules about swimmming that are scattered in various locations.
That's the problem with the rules, there's nothing to describe what happens in-between, or how you get from one to the other, between freely swimming and walking on the bottom. No sink rate/round or anything. You can infer that EVENTUALLY you will sink to the bottom with 16+lbs of gear (although that isn't actually even suggested, the weight is just required to walk on the bottom, it doesn't say that it sinks you), but nothing indicates how to adjudicate that on a round by round basis, which is necessary to counter the distance you CAN explicitly move on a round-by-round basis by swimming. 'You go underwater' doesn't require or imply any change of square[cube], and certainly doesn't require any further movement once you are already under water.
In regards to swimming in full plate, or just swimming with a weighted vest (resistance training in real life), there are two main factors: 1st, the weight being carried while swimming, and 2nd, the resistance that the shape of the armor/vest makes when you move in the water.
For the first bit, swimming in full plate is no different than swimming with 50lbs of any other gear. For comparision, the bag of holding version 4, weighs 60lbs regardless of how much weight is stored inside. In terms of swimming with weight, carrying version 4 of the bag of holding is harder than swimming in full plate.
For the second bit, the awkward shape of full plate regarding movement resistance is supposed to be covered via the armor check penalty. Armor training skills are supposed to reduce the penalties of moving in armor. Encumbrance penalties are supposed to reflect the weight of the armor in relation to the strength of the character.
I saw mentioned the 1st level fighter with swim 1 and 18 strength. At 18 strength, the fighter has a light load with full plate at 50lbs and 50lbs of additional gear. 18 strength means our fighter can lift 300lbs over his head and 600lbs just off the ground - swimming with 50lbs of "clothing" shouldn't be too surprising.
Anyway, a level 1 fighter with 1 rank in swim, full plate (50lbs), a Survival kit(common, 4lbs) and a great sword(8lbs) slung over his back, with 18pts of strength is looking at a light load with no encumbrance penalty from weight. The armor imposes -6 to his roll, but he's at +4 for having 1 rank in a class skill and +4 for his high strength, so he's testing at +2. That said, the DC to swim in CALM water is 10, so he could still fail.
In regards to sinking, remember that since armor affects speed, it also affects how far you can move. If the DM says your sinking towards the bottom, you'd need to factor in upwards movement in addition to moving forwards while swimming. A move action while swimming is a quarter of your movement speed, so even the dwarf in full plate whose encumbrance isn't reduced by the armor, he's still only moving a single square per movement action (5ft is 1/4th of 20ft). A full plate dwarf sinking at 5ft per turn, is swimming up each turn just to remain in place.
Honestly, it's that low strength rogue that won't give up the heavy plunder that I picture sinking to the bottom...
PS: Normal level 1 fighters can't afford full plate. They start with up to 300 gold for all their gear, with full plate costing 1,500 gold.
Whether it makes sense or not, at only 3rd level my antipaladin in Skull and Shackles has "Touched by the Sea" and "Armor Expert for traits, 16 strength (nothing outrageous), and 1, count 'em, just 1 rank in swim. As a result of these things, he can swim in his mw full plate AND with a mw heavy shield and STILL has an adjusted +4 to the result.
Meanwhile, the wizard of the party, at the same level, has nearly died repeatedly from drowning, as swim is not a class skill, his strength is dumped, and he wasn't able to climb the rope thrown to him due to his stats and that not being a class skill as well.
Seems legit to me. After all, two more levels and he'll be able to fly while I'm still running around 20' a round.
I recently had to deal with this in pfs, and a gm couldn't get away from "reality" vs the rules of the game. I failed my check and I was instantly at the bottom. If I ever need to instantly get to the bottom of the ocean while playing with that gm, then I'll remember to just fail my swim check, although I hope he doesn't add in rules with getting the bends upon surfacing.
Did you fail by 5 or more? (CRB p108 states that you only go under if you fail by 5 or more.)
Was the water deeper than 20'? Even with a heavy load you only sink 20' on a failed check (Crypt of the Everflame p20, although this is not a core rule it is a good guideline).