While you can sneak nude at the Diamond Head beach, I have seen cops dressed in street clothes bust a few people. (Although they're quite obvious; they usually travel in pairs and are wearing polos and shorts while everybody else is shirtless/in bathing suits.)
At Diamond Head it can feel sort of like you're in a bad porn, because there are trails directly above the beach and guys walk around them and "spy" on the hotties in speedos below. Most of the time it's not so great, but I have found a couple of six-packs up there and we played together.
If you drive north up to Dilingham Air Field, there's a beach called "Polo Beach" where you can go nude. Note that it's not official, but it's the best place to be nude on the island. Plug the coordinates into Google Maps:
Park on the side of the street here: 21.576851,-158.165505 (68 Farrington Highway, Waianae, Hawaii, United States)
Walk down this trail (it says "public beach access"): 21.577759,-158.165502 (Be careful, it can be muddy and slippery.)
Drop your towel and strip down: 21.580373,-158.164212
If you want some play, there is usually a creepy old guy here willing to suck you off: 21.580443,-158.16337 (or if you have a bf along, you can go hide in the bushes and have some fun outdoors)
Remember, this isn't officially nude but I have been there on Saturdays when there are a good 15-20 people out there (straights and gays) in the nude.
Basically, the way I understand it, unless somebody makes a complaint the police look the other way.