The best snikes in Hawaii (snorkeling to an island and then taking a hike on it) are on Oahu's Windward shore. They are also the only such swim-to hikes in Hawaii, but no matter. These babies are something to write home about for adventure sports nuts. Moloki'i—formerly called the un-PC Chinaman's Hat—is a key feature in a huge regional park in the middle of the east shore. A similar swim-and-hike opportunity is a few miles farther north, at an even-larger recreation area. For a tamer event, try Flat Island, off Kailua Beach Park, several miles south of Moloki'i. Kayaks are in the mix in Kailua.
The water is not deep, but high surf can bring current and marginal or dangerous conditions. A life vest or flotation device won't hurt. Decent swimmers will have no problems—when conditions are safe. For more details, check out Oahu Trailblazer.