Is there a possibility that [atmospheric depth damage] also applies to underwater at some point later down the line?
TL;DRThe pressure damage system is generic enough to apply underwater later, but the focus is first on getting atmospheric depth damage working, and engines functioning underwater is a separate open problem.
The system is generic enough that it can do that. But right now, we focus on that and make that work. And then we can think ahead.
But it's a valid question. But I think there's more of a problem of: do your engines actually function underwater? Because if you sink like a rock, you won't care about the atmospheric pressure if your engines are off. You're still going to sink eventually. There's no getting out of that. Unless we have a ship that has engines that are built for that.
A good thing is we have a system that can apply pressure damage, and it's built in a way that it could even create, in the future, some barotrauma damage effects. So it is actually aimed to have effects. The faster you have a switch of pressure, the more damage could happen, or different types of damage could happen.