Aka a grav vehicle? (Now, the game is unclear on these points, but anything that can use contra grav, handles space (pressure differential) and has self contained life support could do some underwater work (to handle deeper depths, you'd need a heavier structure/hull plus some engineering to deal with higher pressures and different atmospheres. You would probably need a high pressure hull and auxiliary propulsion systems (water jet, worm drive, props) and some maneuvering gear (rudder for port/starboard turns, maybe some other elevating/depressing plane to adjust pitch up and down - Tom would know what they call these bits on a sub) and you'd probably need the equivalent of the tanks subs use to sink/rise.
ATVs can in theory be made to operate in normal earth-like conditions or on planets that have some hairy conditions. In our reality, those vehicles for those settings might be of some similar lineage, but not the same vehicle (because why would you engineer one that can operate without LS, without pressure concerns, without threats of insidious, corrosive or exoctic atmospheres when you are just tooling around on an earth-like planet? You wouldn't... it would drive the price up needlessly).
Sounds like maybe the later case.
The issue with any climb out of a depth is you need to have decompression stops. If you don't, bad things will ensue. Those stops are non-trivial time wise depending on depth. They could be hours or days.