I want to build submarines, boats & underwater bases using the gravship framework. Every ocean tile has an above & below water map. Wall mounted pumps expel water (like Odyssey's oxygen pumps). Timed door airlocks. Diving gear. Different materials for different depths. Constant passive damage to walls from pressure. Harpoons, netted enclosures for fish farming. Kelp farms. Mini subs (like Odyssey shuttles)

Boats should be buildable from wood around a sail module, their equivalent of a grav engine. They appear as a moving caravan, but the boat on moving water is its own small map that stays loaded during transit, so you can zoom in & watch shipboard life. A plank/diving ledge permits deploying/recovering divers from your boat, so you can still explore underwater at lower tech levels.

Maybe submarines are simply gravships with a new addon module that enables them to reverse gravity (pulling them underwater, despite buoyancy)? You'd land on water, a "submerge" option would appear in the launch menu, which would then let you place your gravship outline on the seabed as you would when landing normally.