Hi Jon. I agree totally. A psub should be slightly buoyant with the tanks flooded. We still have the drop weight, but even with that, the sub should float up. That was one of the first things I picked up on when reading the ABS stuff years ago. I don't know how much buoyancy is best, but running a thruster or using dive planes to maintain depth is a necessary evil that a dependable design should include.
My thinking here is that if a complete failure of the blow system happens, and you can't drop your weight, you eventually float up. The most likely scenario for a total failure of all floatation systems coupled with a failure to drop the emergency weight is the pilot becoming incapacitated for some reason.
The one thing missing from this on my sub is the means to open the hatch from the outside. I haven't figured out how to do that yet without having handles poking through the hatch.
I have considered having an external ball valve, that when opened, could be used to insert a handle and crank the hatch dogs open. The trick here is getting a ball valve on the inside too, that keeps the sub from flooding but still allows access to the center crank. Anyway I look at it, there has to be a "block V " seal there. Still working on that one. Frank D.