It's not quite as bad as it's being made out here...I'd say you can expect to earn around $27,000 to $30,000 a year with health benefits. But you'll work 6 days a week for that. If you're willing to live with other folks rent can be reasonable. Check if the shop will allow you to sell photos, that makes all the difference. Some shops allow you to carry your own camera and sell a photo CD. If the shop lets you do this you can clear upwards of $100/day (cash in hand) just from photo sales. You won't make mogo tips here, but you'll pay for lunch and dinner from tips if you're a likeable person.
The diving is all over the place. I've done some of my best and worst dives on Oahu. There's LOTS of turtles, way more than the Carib. The coral is pretty unheathy, mostly because of dive operators abusing the reef (though there's also less coral growth because of the lower mean temp). There is no soft coral or large sponges to speak of (if you've done Fla/Carib diving).There tons of fishing too, so some sites are overfished to the point where there's nothing to see. Water in the summer is cool(79) and in the winter it's freezing (around 74 really...you've just got to wear a 3mm or a 5 if you get cold easily). The vis here ranges from 3 feet to 100+. The winter time has whales (you can see and hear them on dives). You can expect to see a good showing of fish and invertibrates, but not tons of sharks or rays (though some sites have regular sitings of both). Surface conditions are generally rough and you see a LOT of puke.
Lets see...traffic is bad, but the bus is cheap and reliable. Food is expensive, but as was said earlier there's Cosco and Sams Club. Local are nice, friendly people so long as you are respectful of the culture. Shirt and shoes are generlly optional. Surfing is great but crowded, and there is some really good (but tough) hiking. If you are into golf there's lots of courses. If you like to hunt the forrest is full of pigs who don't belong here. There is a surplus of VERY attractive people here (if you're single) of both genders and orientations, whatever floats your boat.
If you end up living in the Manoa/Makiki area Magoos is a cheap but busy bar.