My favorite part about this module was dealing with the environment. And it's not necessarily even that hard -- I just really enjoyed the chance to hear what our problems would be (it's no spoiler -- right in the overview text at the top of this Web page it tells you that you're going to the plane of water) and then as a group we all started figuring out what we could do, what we could contribute. Apparently the module expects players to spend a chunk of time (and money) on shopping & planning, and so it even sets aside time for it. People were looking up spells, finding magic items, etc. It was nice to participate in teamwork in a no-pressure, no-combat situation.
Table Variation
One thing your GM should clarify right up front: how do monsters and polymorphed PCs (and companions, and summons) qualify to do full damage in water? The rules for water state that "land-based creatures" have to do half-damage, but what counts as land-based? It's not a creature type, and not officially defined. So for example, if you play a druid and you wildshape into a water elemental... the elemental form is a water type, but the original caster is probably land-based. So does that druid do full damage on a hit (due to being a water elemental), or half damage (due to the PC's land-based origin)? Does being the aquatic type confer full damage, or does living underwater your entire life confer full damage (even if you're not aquatic)? From what I can tell, this is not explored deeply in the GM forums, nor the Rules forum. So expect table variation.
Therefore, expect that you may have to change your game plan. Depending upon what your GM says, you may decide that you'll focus on wildshape, or focus on summons, or focus on your companion. So GMs & players both: get that resolved at the start, so nobody is surprised mid-combat.
And GMs: be careful how you rule on that, because I know of at least 1 way to rule in which all your monsters do half-damage, too. So when you come up with a ruling, make sure it's not so punitive that it wrecks everything across the board. If you're too harsh, the players may hold you to it when it bites you back.
Also, the official rules for many spells when cast underwater is that they "don't work as expected" and you may need to ask your GM. So that's another thing you may wish to clarify up front. Do my lightning spells remain in a line? Do they begin to take on a sphere shape? Is your GM deciding that since the rules don't explicitly say how the GMs may rule, that he/she won't rule at all and everything is exactly the same? Find out beforehand!
So what's good?
OK, so why is this fun? Aside from what is essentially a mini-game to defeat the limitations of water, the story actually furthers the season's plot. I got to hear a lot about the lords of the planes, and have an effect on the outcome (granted, any group that succeeds will "have an effect on the outcome" but at least we're rolling the plot forward). The combat encounters are decent -- they were hard enough to damage us, but not so hard that the team gave up or players got frustrated.
Well, wait a second. One fight was hard. Like really hard. However, advice for GMs: if your players are in high tier and there are only 3 or 4 players at your table, be sure you give them all the advantages listed. Our GM was like, "Wow the 4-player adjustment is really helping you." Then we had the fight. Let's just say that we needed the help.
This game is probably a planner's paradise. Not only can you plan out how to handle water, but in at least one case the PCs become aware of an enemy before combat is engaged, meaning that the PCs can calmly put together a game plan for handling the encounter. Or rush in without planning!
Also, there is a big choice/consequence check in the game. I can't detail much without posting spoilers, but the general idea is that how you approach things and the balance you have between speed and caution can affect a certain someone's impression of you. When we experienced this, some players wanted to take back certain decisions, or convince the GM that the group could still fix things after the fact. But that isn't how the adventure works, and honestly I was grateful for it. I like that you are making choices that you don't even know will have an effect, and you absolutely get an advantage either way, but you can't get both advantages. Here, I'll spoiler this just so that people who have played it can match up what I'm talking about:
Overall, very fun. Not perfect, not 5 stars, but very good. There is that 1 fight that I suspect will be too hard for a few groups, and as the previous review noted, the game can run long. Set aside 5 hours for this one, maybe even more if the players are exploring every little thing and the GM wishes to run all the optional stuff. So I think this adventure is pretty great and if we take 1 star off for the too-hard fight and a few other nitpicks, we still have a 4-star product. Not bad at all.