RE: Newbie here, how does it all work?
All combat ships have a fiberglass hull 1/2" below the waterline. Above that, builders are restricted to Balsa with thicknesses dictated by the rules. Submarines would not be much different.
All the submarine "sinkings" at that time were referee sinks. A judge made a declaration based on round proximity.
Both of the M-1s were piston bouyed, and there was no room for penetrable areas.
With a dynamic submarine, dry hull, it is possible to actually create the conditions for sinking, but we have yet to recruit the talent to produce such a boat. And the speed and dynamics of the game are not conducive enough to promote anyones interest. It is so easy to outfit a DD, that everyone takes the quick route.