Whew. One thing for sure is that you should NOT expect to run around much on the first days of your honeymoon after throwing a rehersal dinner for 80 and then wedding for 140... ALL FAMILY. What a blast.
BUT, this is about scuba... we chose great resorts (Ti Kaye
http://www.tikaye.com/) and Sandals Regency. Ti Kaye was private and quiet. We slept and snorkled for three days. Sandals Regency (we also visited the Grande) was more active and of course populated. As soon as we switched gears from "Ti Kaye Honeymoon" to "Sandals Spring Break" speed, the Sandals resorts were fun.
While snorkling at Ti Kaye I found that the experience wasn't much different from our home beaches in Lauderdale by the Sea. The Sandals cattle-boats dropped their resort divers and snorklers in our cove twice a day, so I got a look at their ops from the land and water first - turned me off to THOSE experiences, so we didn't dive at all. It just didn't look like something I needed to do, since nothing on the extensive snorkling trips I took was enough of a wow... compared to the honeymoon, of course.
Repeat: I would **NEVER** book a dive or snorkle trip through Sandals.
Transportation on the island is expensive and time consuming although I'm sure that off-resort dive ops would have been worth the trip, especially if you don't get to dive the southeast US/Caribbean much... we just didn't try.
Sorry I don't have any success stories or must-do's but - REPEAT - if you dive at all in SE Florida, you can pass on the Sandals dive experience. There... at least I saved somebody $70/dive and most of a day.
Summary: St. Lucia is awesome and the people are great with a low crime rate. Everything is so clean, even though its a poor island. Cabs are very expensive and there are no direct roads... everything winds around volcanic terrain. If we go back, we'll do a non-resort trip, but Ti Kaye is perfect for the first few days of a honeymoon. I don't regard St. Lucia as a primary dive destination, however, since islands like Grand Cayman and Cozumel are soooo much better. Just my $0.02!