I can't help on water and air temps, although I can tell you that I was there this past August and we were wearing 5mil full suits. It really depends on where you are staying as to what clothes to bring. It can rain so I would recommend a windbreaker or light rain gear. If you plan on hiking, bring some boots as the hills can be rugged. Don't go overboard on clothes. I think you'll find that laundry is cheap and available...and you'll most likely be in your bathing suit, or wetsuit, most of the day anyway
As for gear, that also depends on where you go and how much you trust the rental outfit/s. My wife and I brought our own BCD's, regs, computers, fins, masks, snorkels, and light cannons. However, we normally travel with our gear anyway. We rented wetsuits, for the sake of convenience. I would also recommend a safety sausage and/or other signalling device as currents can be swift and chase boats don't always necessarily chase. Most resorts will have fins and snorkels for you to use..but I'm always more comfortable in my own. And we did A LOT of snorkeling...and a dozen dives.
I suggest calling ahead to where you are staying and seeing what they have to offer.
FIJIAN CUSTOMS: Kava drinking is a must. There is not much to know about it...except that you have to drink a tub full of it to feel anything other than a fuzzy tongue...
You'll soon learn the value of "Fiji time". If you've been to Mexico you already know the routine.
Learn to say "BULA" it is hello, goodbye, and all other greetings combined. Try not to be too flashy, especially around villages. And don't enter villages without being invited. If you are invited for dinner at a villager's home they will wait for you to finish your meal before they eat. So don't take too much food.
The fijian people are among the nicest in the world...no worries.
American customs/tendencies to avoid(there is truth to some of these
):
1) Bermuda shorts with dark socks and topsiders/loafers
2) Complaining that there are no good Sushi restaraunts around
3) Complaining in general
4) Overtipping
5) Being upset that there is no TV in the room.
About the hat: yes, wear the hat, but please, not while diving.....
BULA!!!
have a great trip...
and let us know where you are going....