A book by Franciso B. deCarvalho, "The Oahu Snorkeler's and Shore Diver's Guide", is a starting point; available at Amazon, if not at your local big-box. The best bet for shore diving is Kahe Point Park and the nearby Electric Beach; both are usually divable when the rest of the island is snotty. Both are in the shadow of the Kahe power plant; can't miss it, just get on the freeway west (toward Waianae) and go until you see the smokestack with its strobe -- a 20-30min drive from Honolulu, depending on traffic. Kahe Beach is a shallow dive, not more than 20-25ft, and is very popular with diving classes -- meaning it's gonna be crowded on weekends and especially after 0900. The entry/exit through the little cove can be "sporty" if there's any surf big enough to produce whitewater. The main problem is that the safe route is a 10ft slot between boulders next to the power plant cooling pool and a flat rock shelf on the park side. The exit usually gives the most problems because the surf typically breaks/plunges right at the entrance to the cove and you can't see (and can't really swim) in whitewater foam. I can't tell you how many times I've seen people swim in, only to get their feet picked up by a wave and then sent "over the falls", head-over-heels into the rocks -- pulled more than a few out myself and found a boxful of gear that got torn loose. I use a funky technique to exit through such surf; it basically involves tucking into a ball and flipping so that I'm headfirst into the wave before it hits (and flipping back after it passes to continue swimming in); crazy but it works. Again, if there's surf, take the time to watch others do the entry and exit. And besides, because it's so shallow, viz and surge is going to suck if there's surf. But on a good day, Kahe has a lot to offer. Reef whitetip sharks resting under overhangs, the very rare Hawaiian flame angel (a brilliant scarlet), turtles, octopi, harlequin shrimp, squeaking dolphins further offshore, are possible. Be on the lookout for the peacock flounder which is almost impossible to see and can disappear in an instant. As far as flora and fauna, Hawaii is poorer in quantity and variety as compared to spots in the Carib or South Pacific. Hawaii is subtropical (cooler waters), extremely isolated (low species count but very high % of endemics), with surf and currents which favor stony corals over soft-corals such as seafans. See Francisco's book for other spots. The main cautions are surf, currents (always, always watch it for 10-15min and be absolutey sure and honest about your abilities because Hawaiian surf is probably faster and more powerful than you're used to), and crime -- lock your car, carry no valuables, don't park in some far-off place.
