I think would be nice ships and oil rig to have a internal space from factorrissimo tanking advantage of it code.
So we can research FPSO ship to get the internal area, and Oil Platform to get oil rig internal space.

I think that could be the subject of a different mod, maybe a water buildable factorissimo. Maybe one can get inspired by something like https://mods.factorio.com/mod/Spiderissmo, but I personally won't work on that or implement it into this mod (I I don't think it fits in the base mod)

I think I saw a mod recently that gives normal trains an "interior" where you can walk between wagons and everything, but I can't find it now. But yeah, it sounds like it should be a separate mod if it works at all.

Found one like that:
https://mods.factorio.com/mod/Immersive_Locomotives

Thanks! Immersive Locomotives actually already works with Cargo Ships, if you can get close enough to enter the body of the ship with the normal Enter command. It does not create internal surfaces for the Ship Engine, and the Shift-Enter command doesn't let you enter the body of the ship right now.

A compatibility option could be added to that mod to make the internal surface for cargo_ship and oil_tanker larger than for a standard wagon.