It's important to read the local newspapers. During our trip to Maui last August, we learned that certain beaches were closed for a few days because of shark sightings off shore. They were D.T. Fleming Beach in west Maui and Baldwin Beach, near the Kahului Airport. Since then, I've been wary of a deserted Hawaiian beach we could have all to ourselves and currently prefer a beach with people on it. There was one shark bite incident - a windsurfer got bit about a quarter mile off shore at Hookipa, the famous windsurfing beach near Paia. These things seem to go in phases.
General consensus is a snorkeler usually is (1) in shallow enough water, (2) close enough to shore and (3) part of a larger group. These factors insure we will never come close to being shark food. I'd be fine for a half hour, then keep wanting to look around to see if anything was approaching. One unsettling thing: occasionally there will be a pod of dolphins out there. They're fine if you're on shore and have some time to figure out what you're looking at. The dorsal fins cut through the water, then suddenly one or more dolphins will jump up and splash back down. They'll disappear for a bit, then one will break the surface and spin around. Generally, they're not all that close to shore, but you can tell there's some big animals out there. If you have someone from your group in the water when you first spot the fins, it can be pretty exciting. Your brain tries to override those scenes from 'Jaws' that automatically start running through your mind while you attempt to figure out what you're really seeing….