I love Anse Chastenet and their upper end hotel, Jade Mtn is one of my ALL time favorite hotels. You have a pool INSIDE your room, private butler and only 3 walls on the room. BUT the diving in St. Lucia has deteriorated to an algae covered bottom with a smattering of fish.
How about Fiji? Here's why. You leave on Thurs. sleep on the plane and arrive Sat. am (out of LAX). There are amazing resorts for an anniversary. I highly recommend Wananavu Beach Resort (they were just running a great special at the Long Beach Show- email and ask them). They have brand new honeymoon villas and one (bure #50) is ON the beach with it's own private plunge pool. The diving out in the Bligh waters is some of the best in Fiji. Only thing about August is that it can be windy (or not!).
There are other great resorts on Fiji as well, but that's my fav. There is a new zipline course being built just outside Nadi, I think it is supposed to be finished in July. Or, take the Long way around the island through Suva and stop for a night in Pacific Harbor. There you can do the amazing shark dive with Aqua Trek (bulls, tigers, the works!) and also do a day of rafting and a zip line over there too. LOTS to do!!!
Anyways, that's my vote.
Oh, if you go the Belize route, I highly recommend Turneffe Island Lodge. Very quiet on it's own 20 acre island, great diving 5 minutes from the resort. You can also tag on a few days at Blancaneaux, Frances Ford Coppola's beautiful inland lodge, or if you like to bird, try Chan Chich for a few days inland. (you can do four days at Turneffe and a couple at Blancaneux- though it makes it quite busy. I'd try to take 10 days.
Personally I think you can do much better than Coz or the keys for your anniversary.