Rules for a submersible?

[Corrupted Valor]

I was wondering if there were any rules or ideas people had on rules for submarines and underwater siege weapons/vehicles. I have a gearhead (3pp Pure Steam Campaign, it's like a mundane arcanist) player who wanted to build one (I house-ruled vehicle creation as an item creation feat) and just on the fly threw together something based on the steam wagon(PSC again) and a boat with a couple of light ballista substituted for harpoon missiles.

Searching around I couldn't find anything, so I was wondering if anyone here had any ideas.

Hell's Rebels Part 3:

They're headed down into the eye and wanted to design a deep sea vehicle to help with the pressure, and they really didn't want to deal with underwater combat again since they don't have traditional casters, and no one with divine other than a paladin 4/swashbuckler 3

[Ridiculon]

Apparatus of the Crab is the only one i know about in the game

[Corrupted Valor]

Unfortunately it doesn't use the vehicle rules, it seems like that functions more according to the monster rules.

[Cevah]

An animated object could satisfy what you want. Per the info on the linked page, you have two options: Permanency + Spell, or Craft Construct. There is a limited Craft Construct type feat that lets you make only one type of golem, without the required Craft Wondrous and Craft Magic Arms and Armor. You could re-flavor that limited feat to be the one you gave. Therefore they can only make animated vehicles and no other types of constructs.

Under ship-combat and subpages, are a bunch of rules you can peruse. More can be found for aquatic-terrain.

Biggest thing I can think of is that ranged combat is hard, and imposes a big penalty for distance. Melee combat may be hindered, but FoM can help with that.

/cevah