I was at the island this evening
(well, I was there all day...er, but whatever!) and I found at
least 5 sharks in 20 mins or so. Otherwise, I've found several on my town beaches around the afternoon/evening.
I've been shocked to see how easy fishing has been compared to WW. I already have sharks, a dorado, and probably whatever else is considered "rare". I've had my WW town for 4-5 years now and I still haven't completed my collection. I have a hard enough time finding "finned creatures", let alone catching any. I've completed over half of my NL collection in 3 DAYS. DAYS! Completing my fish/bug collections before my fossils? I don't know what I'll do with myself.
Anyway, I already have all 4 sharks. I've missed several, but it definitely helps to listen. I make sure my volume is at max when I'm trying for a shark. It just takes practice.
Maybe try closing your eyes like Sleepy said.