There have already been suggestions for both boats and submarines. One question to ask with it is, in which way will these, and other vehicles, carry people inside? For example, i would like to have a small boat, a little fishing boat. Sure, that would be a vehicle i can simply sit in and row it left and right. But if i would, for example, want a big boat (or submarine), that can carry me and my friends, it would have to be quite bigger and more advanced. This here is my suggestion. It'd be awesome, if you could have a boat, that you can either steer, or also walk around in, as a passenger. Like a yacht or sailboat. Up to even bigger sizes, say, Queen Marry-ish, if you can find a big enough ocean planet to ferry. Examples from a different dimension: Being on such a ship would firstly have a social purpose (hanging out with friends, chilling, chatting, doing calm activities like fishing, swimming, diving and such). Some non-serious relaxation is often fun, look at Second-Life-ish games. These boats could actually be a fun social hangout place, especially if it's of a somewhat big size, so people can walk around in it. (Oh god i'm demanding a lot here) However, it may also be a part of the game in a useful way. You could equip your ship with a sonar to hunt underwater treasures or equip it with arms and cannons to fight against whatever you might possibly meet on the big blue/orange/green sea. Think about the boats 'n Floran hoes.