Dawn is a miracle chemical!   I've used it on a few things besides dishes. 
Sharks never bothered me, but barracudas are psychos.  They'll hit anything shiny no matter what it's attached to.    I've lost fish to them; most notably fighting two lb albacore tuna on the Coast Guard pier in Key West when the line released like it snapped.  It hadn't snapped; I had the head of the fish, cleanly cut off at the gills.    Whenever I hooked a barracuda (small ones), I cut their heads off and them them back in.  Not a fan.   Sharks are not a problem.  Most were Lemons and Nurse sharks. Bulls and Makos could be dangerous but I never saw one when I was in the water with them.