[hteas]

Naiad,
Your comments bring up another training technique.
I do dynamics with partially full lungs. I become neutrally buoyant by exhaling until I can swim without having to fight to stay down. I do repeats of 25 m on the bottom followed by 25 m on my back on the surface for recovery. They're very conservative, but if I ever did push too hard, I guess I'd just float there. Seems a little unnerving. Anybody else train the same.
No, typically no partner-I have yet to find a reliable one, so my choice is to train conservatively or don't train.