Mark, I'd recommend taking a water taxi to Caye Caulker rather than booking a day trip that stops there. The taxi gives you more opportunity to interact with Belizeans, rather than just other tourists. There are plenty of outfits on Caulker that can take you on a snorkel trip in the morning - hopefully the Xanadu folks could help you book that. You'll have the afternoon to find a place for lunch, wander the village and hang out. Caulker is far slower than San Pedro - a day on Caulker will totally get your internal stopwatch reset to vacation mode.
Another snorkel guide to consider in San Pedro is Alphonse Graniel - do a search of this board to find a lot of comments on his tours.
We'll be on AC 4/2-4/9 and we try to stretch the week as much as we can - redeye into Belize in the morning, on AC by noon and when we leave, we make it as late in the afternoon as possible. And, every trip, we wish that we had more time. The good thing is that there's always next trip to look forward to. That longing for San Pedro begins about 5 minutes after the plane leaves Goldson....