Submarine Science School Demo

A reader writes in and says:
I "volunteered" to do a science demonstration for my son's elementary school. When they found our I was an ex-submariner (642B) they requested something sub related. I've thought about the following:
**steam cycle demo, but am having trouble coming up with a way to make steam safely at a high enough pressure to turn an old computer fan (simulating a turbine)... the whole condenser part would need to be on the white board.
**buoyancy demo using pen caps in a sealed 2L bottle - squeeze the bottle, increase the pressure, reduce the bubble size, cap sinks... lacks a certain pizazz
**the old blindfold them and let them play submarine hunter with wet sponges sonar demo - fun, but lacking ideas on where to take it after the first couple kids get smacked with a wet sponge.
Any ideas - either improvements on the above, or new ideas? (I'd like to keep the cost under $50.)
Every idea I came up with -- except for the old "raisin in a glass of water with vinegar and baking soda" idea to demonstrate buoyancy -- cost a lot more than $50, so I figured I'd throw it out to the Peanut Gallery for comment. Have any of you ever done something like this? What ideas do you have?