Interesting idea Si!
I don't think I would go for it because of the affect on buoyancy - it would annoy me! Also, I would object to all the hundreds of pence wasted in gas fills...
I think another interesting solution would be an automatic deployment based on run time. You tell the system how long you will be on the bottom for. When that time has elapsed, you get an alarm. If you don't shut it off, it fires the gas (solenoid in loop or wing) and up you go. Hopefully you will be in the region of your boat and ....rescue? Hmmm...
As somebody once said, "Insurance companies like to have a body..."
