I just got back about 2 weeks ago from my honeymoon in St Lucia. We stayed at the Regency it was cheaper and had better food! Although, 5 out of the 7 days we spent the entire day on the beach at the Grand. The beach at the grand is much much more calm and the water is far clearer. Make sure you do the included water skiing and wake boarding as well as the catamarans its all a blast!!! Make sure you also go to the french restaurant at the Regency its the best on the island! You have to make sure you get reservations the day you get there for all the restaurants that require them they fill up very very fast and they wont let you in!
As for the snorkeling and scuba we didn't want take the time to get out certification for scuba but we did go snorkeling the waters are shallow enough in most places for a health person to be able to free dive and look all around!
The mistake we made was under estimating how nice it was going to be. I was thinking included snorkeling yea that's going to be here is a pair of goggles and snorkel go out on the beach and poke around. So, I didn't buy the waterproof digital camera my wife wanted to get from bestbuy.
No, you have to make reservations at least 24 hours in advanced the boats leave at 8am 1pm and 3pm and the 8am is mostly for the scuba because its way longer then the other two. You ride on a big boat for about 45 min up the coast to one of several spots the water is crystal clear and its a lot of fun you will love it. I thought you might like it so here are some photos and a video!
You can see one of their boats in this photo
At the grand
Sunset at the grand
view off the cliffs at the Regency
The boat closest to you in this photo is the scuba boat
Landing