Loved it. Something great here. I would say that the biggest issue you may have, and I could be wrong, but I feel like you are giving TOO much in the demo? Obviously I don't know what the modifiers or other characters bring - maybe there's a lot of replayability provided by those. Maybe there's a lot more after surfacing, but based on the name of the game, I'd assume the surface is the end?

Not that this matters at all for the sake of the gameplay, but as someone that lived on a submarine for 5 years, some of the requirements for each depth only really make sense if you are descending. You'd need a stronger hull the more pressure there is. Glass on a submarine is a scary thought altogether but the same concept would apply. But again, these wouldn't deter me from enjoying the game.