Sorry. I was just ecstatic someone agreed with me. You're right I should have done a better job of that.

Anyway, the downside is I finally got hold of an engineer I know who has a maritime background. He said paddle wheel thing is about your best bet as you wouldn't have to compensate for density: an inch of water is an inch of water. You could have inlet in forward facing torpedo tube and outlet in aft facing tube. You could also shift it to a non watertight compartment, creating room in bow, if you can find one that is submersible.

He also suggested a way I havent' seen posted and that is to use and accelerometer. True it would only work in still water but there are several phones that have internal accels built into them. Just a thought.