There are quite a number of 'possible' solutions to getting a boat back to -a- bank. Getting it back to a specific point on -a- bank complicates things by several orders of magnitude.
If this system works when there is a loss of signal, would it 'kick-in' when ever you stop the boat still. Distinguishing between intentional and accidental loss of signal would certainly be nice. There's no constant signal with the typical R/C radio system, it depends on the operator issuing a control movement, it's intermittent. Redesigning R/C radio systems isn't very practical an idea for a number of reasons (battery size?). A waiting period before activation? I can see problems with that for IC powered boats and 'fail-safes', how do you restart that @#$ IC engine?

Please don't misunderstand, I think you have a pretty nice idea. I honestly don't think it's too practical right now, and for R/C'ing, but it certainly does have applications! I wish you luck with it.
I'd drop hints about you letting me do the marketing for it, but you ain't that dumb to let me, so I won't...RATS!
- 'Doc