I can't remember which interview it was but Sean tells a story. He was asked what his most memorable death was. He said he was exploring / testing on a new planet. He had a waypoint / location of interest marker on his mini map, he went to it and it was under the water in an ocean type area. Ships cant go underwater, his suit was fully upgraded (may not have directly said this) but he wanted to know what was down there, he swam, swam and ended up dieing before he could get down there. He will never know what was down there, and was really intrigued. He said the procedural generation shouldnt have put an item of interest that deep down, but it did. He will never know what was down there. If he could upgrade his ship / suit enough by just using the dev tools, im sure he would have and just checked it out. Maybe he did after but wants to let us know that sort of thing can happen. Either way its interesting.
Ships can't go underwater but Sean said that there are other vehicles in the game such as land vehicles. I'm not sure if there's gonna be a submersible type vehicle.
If not I'm sure it will get a lot of requests by the community and they'll add it in the future.
Aye Padrino, Sean has said before that they will continue to work on the game and see what people want over time. I think submersibles could well qualify as a potential addition at some point in the future. And probably fairly easy to implement.