Complex sentence: Early sporting Fathometers for recreational boating used a rotating light at the edge of a circle which then flashed'synchronized in time' with the received echo (corresponding to depth); these also gave a small flickering flash for echos off of fish.
Split into simpler sentences: Early sporting fathometers for recreational boating used a rotating light at the edge of a circle which flashed in sync with the received echo, which in turn corresponded to depth. These also gave a small flickering flash for echos off of fish.