Silty Sam
Contributor
Clever, and looks like fun, but it doesn't solve the problem of how to mark specific waypoints, such as if you find that anchor again.
Yes, those are problems. Possible solutions - one I've tried, one I wonder about - include mounting or attaching the phone or GPS receiver in such a way that it could be pulled underwater to stop receiving the GPS data.
And when I've tried this, just stopping in one place for an extended period and pulling the tow line taut so the buoy/float is overhead makes an "enlarged blotch" on the map so you can sorta see the specific waypoint.