I know after about 7pm is a good time but for whatever reason around 4 am I kept running into them over and over again. Is the spawn rate better before dawn or was I just lucky to have so many appear?
Any time past 9PM is best, but not because of any different spawn rates - simply that past 9PM the Ocean Sunfish stops spawning, so you'll know for sure that any finned fish you see while catching beetles is going to be a shark.
On the flip side, 5PM-7PM is the worst time, because of all those grasshoppers and long locusts hopping around, and they're much harder to scare off than hermit crabs and wharf roaches.
I like going past 11pm because of the chance of catching giant stags and scarab beetles... I just have to TT to that time, because I'm never up that late! XD