Originally Posted by Nemo
Make the cylinder from polycarbonate. Its clear, tough and readily available. Best of all it will let you check the operational status of your gear at a glance.
As to sinking, that's an interesting one. A wet-hull sub does not rely on the hull for keeping the water out. Its just there for cosmetics and hydrodynamics. You'd probably need to skin the inside and use at least a portion as a watertight area that floods when damaged. That will take up room for all your fun toys (torps, etc), making this an even more challenging build.
Perhaps you could rig up a trigger to the inside of the hull that would, when breached, vent your tank and sink your boat? That way once you're sunk you can just throw a toggle, blow your ballast and return to the surface for recovery...
Well, perhaps insted of using foam to add boyancy to the upper portion of the sub you could use air filled compartments instead. They are already at the waterline and are penetratable. When enough are holed, the sub sinks, and to repair, just blow the water out and patch the hole. One problem might be how this effects trim though, a leaking bow compartment could make the sub uncontrolabel after one hit, even if it does not sink.
Again the only problem with this that I can think of is if rules prohibit that type of construction, tho I can't imagine why as long as you can sink it.
A better idea might be to build it LIKE a wet hull sub, but make it a dry hull, as it leaks it is still controlable, but this would mean lot of extra ballast to fight the boyancy. Also easy to repair.