We did most of our research using books. Probably our best resource was The Book of Submarines by Jack McKay. There also was a video on submarines that had a lot of glossary words. It was difficult to find good books. A lot of the big, thick books complicated but the little books were easier to understand.
Submarines have compartments on the sides.
When they want to dive they let water in. When they want to surface they push air in. If they let some water in it goes a little bit deeper. If they barely have any air in the tanks they are barely touching the surface. This way they can get to lots of different depths.