Show HN: I built a sonar into my surfboard
As a surfer at crowded breaks, I've often thought it would be cool to build a little drone submarine early warning system - it would navigate itself out a half mile or so offshore, just under the surface. Then, when a big set comes,  I would get a little buzz from a smart watch or ankle band, and casually paddle out past the crowds and get my pick of the good stuff :)