There's good news and bad news.
Good news: The sub is neutrally buoyant, and stays upright in water. The design seems solid, moreso than I expected on the first try.
Bad news: The RC minisub that provides maneuverability is kind of crap. It has no ballast tank and relies on downward angled props to descend. This barely works. I can descend, but not without going forward a bit, and the minute the props stop spinning, the sub slowly ascends (which I can accelerate by reversing the prop spin direction, but the sub also moves backward, etc)
Solution: I've ordered a replacement mini RC sub that is much more expensive but which has a proper ballast tank, so it can greatly alter its own buoyancy on command. It also has stronger thrust, as you can see in the video that's something Hamsub sorely needs. The current thrusters struggle to make the sub turn against the influence of the air hose, which is as thin and flexible as it comes.
Here's a vid of the new RC sub that will soon be providing maneuvering capability for Hamsub Mk.II: http://www.youtube.com/watch?v=cWbuaKVH-JU#t=0m36
I expect the final product will also make an excellent low cost ROV simply by sticking a video camera inside the passenger capsule. For once, one of my projects has practical applications. Feels wrong somehow.