Just got back from a week long trip to Ambergris Caye and Placencia following Labor Day weekend.
We dove with Island Divers in Ambergris/San Pedro and really enjoyed it. It being the low season, my GF and I ended up getting pretty much private boat tours on all but the first day (really appreciated that they ran the dives regardless of boat size). Additionally, I'd note that the dive masters were really flexible in scheduling as we would wait till the evening to check the weather and decide whether or not we were going out and where; definitely a more personalized service then the larger shops on the island.
For me highlights included:
Hol Chan and Shark Ray Alley: Shark ray was a snorkel with chum off the side of the boat, tons of fun. Hol Chan was pretty much empty when we got there, lots of spotted eagle rays to see.
Tuffy: Pretty impressive and deep set of coral, we started out fairly shallow but got to about 90 feet or so midway through (although could have gone quite a bit deeper)
We lucked out with the weather (sunny and 80's every day), but I did hear it was supposed to rain for a couple of days as we left the airport.
We didn't dive in Placencia just spent a couple of relaxing days, only mention it as the sea grass smell was definitely not as bad as in San Pedro. If staying in Ambergris, we'd recommend staying at Mahogany Bay as the resort is free of it, and it has a private beach that was pristine (if you stay elsewhere the only other option is to go to secret beach, which from what I heard was packed)
Pictures to follow as soon as I get a chance to edit.