Tagged, I for one, like the idea of using accelerometers to monitor / calculate position.
It doesn't really work because of current drift. Or, rather, it does, but the gyros required need a submarine to house 'em. Or an ICBM.
What you could do is log the IMU readings along the way, and GPS coordinates of your start and end positions, and then try to back-fit the calculated trajectory after the fact. At that point you're back on the boat already, so here's little practical use, but until someone starts doing this, we won't know how (in-)accurate the cellphone/dive computer IMUs really are.