In
Silent Hunter III, you dived your boat by hitting 'D' (devastating on high time compression
). If you didn't level out after hitting the Dive key, you'd eventually either hit the bottom or get your boat crushed by pressure. So in essence, 'D' took you all the way down, 'S' took you all the way up. 'A' levels you out.
Try the same with this game. Hit 'D' and then 'A' when the boat starts creaking too ominously.