Re: How do submarines sink and then float again.

Submarines use a concept called negative buoyancy. If buoyancy is being 
lighter than the water displaced by a vessel then negative buoyancy is 
being heavier than the water displaced by a vessel. By allowing water to 
enter ballast tanks the submarine becomes heavier and sinks. When the 
submarine wants to surface again, the water is pumped out. Also submarines 
are moving forward because of their propeller. They have "wings" on their 
sides that they use to go up and down in the water like an airplane. Learn 
more about how to construct your own submarine at these websites. Good 
Luck!
www.submarine.freehosting.net
www.history.navy.mil/branches/org8.3a.htm
www.unmuseum.org/exsub.htm
www.yellowsubclub.com

My personal advice is to keep your design simple!