RE: RC Shark
Good idea, you have to be aware that some ballast tanks allow you to dive the sub at a certain depth and "float" with a bit of the sub on the surface, a pump ballast tank is one example. This is a good starting point because if you have failures in your diving planes, the sub will still float up because it is slightly buoyant, just adds a bit of realism because you're diving vertically.
Some complicated ballasts like a gas powered one will aquire more experience. A very good choice will be a piston operated ballast tank because it acts like a sucker to suck water in and out, no gas required!