Perhaps knowing more about the project's rules and limitations would
be helpful but how's this for another suggestion:
Develop a "diving chamber" that would be filled with some material that
would dissolve in water (salt or sugar may do).
If the rules allow it, do the following: fill the chamber with the
solid material and let the chamber first float, then fill with water through
vent holes. When enough water leaks in, the chamber would then sink to the bottom of the water tank. Then, after the sugar or salt dissolves and
is allowed to trickle out of the chamber, the chamber could then rise
to the top (as it could have air bladders that are competing with the solid material to achieve positive and negative buoyancy).
The basic idea comes from smugglers who drop contraband packages into the ocean, weighed down with salt blocks - and the packages rise up after
a certain time to be picked up. I saw this on TV, not gathered from personal experience