On Oahu, for a novice, Hanauma Bay is probably the best bet. Getting there around 8:30am should be sufficient to find parking; the admission lines move fairly well; check the website. There's a spot called "Sandman's Patch" just off to the right of the old telephone line channel. It's a little less crowded and more fish pass through the area. Don't go past the marker buoys into the deeper areas of the Bay unless you're a confident open ocean swimmer.
In Waikiki, the areas fronting the old Natatorium and Waikiki Aquarium can be good snorkeling.
Another Oahu spot off the beaten tourist path is Kahe Point Park which is a bit of a drive west of Honolulu. Be warned that there's no lifeguards, it's open ocean, and vandalism/crime can be a problem. But because of the warmwater discharge from the nearby powerplant, it supports an unusual profusion of fish and corals. Deep water pelagic fishes and dolphins occasionally pass through.
On the kona side of the Big Island, you have to work to find a bad place. Even the area fronting the Kailua-Kona pier can be worth a dip. Pawai and Kealakekua are both winners.
On Maui, people tend to take a dip at Olowalu, which is just past the long straightaway after the winding drive along cliffs just past Maalea Harbor, on the way to Lahaina. Go there only during daylight and when the water's clear; it has a sharky reputation.
Check out the Hawaii O'hana forum on this board for more ideas.