How about a robotic arm to grab things and bring them to surface. This could actually also be a good mission for the OpenROV. One could actually retrieve objects someone has lost, in a lake for example.
how about this arm? https://www.sparkfun.com/products/11524, for 12$ it does not look bad
the only problem is that you need a servo that can go underwater...
For $12 it is worth a try. Has anyone tried filling a servo case with oil? Mineral oil or peanut oil should do.
Perhaps we could use the hydraulic design, but instead of using pressurized air, we could use oil-filled syringes, acting like small hydraulics. For actuating them, a separate capsule would contain a single syringe and a stepper motor or servo that spins and pushes the head of the syringe, which is ported outside through a potted exit port to the gripper.
Steppers would be controlled with these, and I hope there are enough I/O ports left on the 'duino to support it. http://www.pololu.com/search/compare/120