Thanks for the suggestions.

OK - I've gotten pissed off with the awkward nature of the physics engine so I wrote my own. It works really well and the sub handles naturally. The only thing it doesn't do is transfer momentum to other objects on impact but I don't really need that anyway.

I tweaked my physics so the boats really feel like the real thing. I've got a few work-mates to test-dive the sub and they love the feel of it. I've adapted the physics for both underwater, surface and airborne (for air-drops).