Experienced snorkel in Belize?

We're experienced snorkelers that will be in Belize on the Carnival Conquest in late May. We have our own snorkeling gear and aren't usually interested in being lead around by a guide. But Belize is new to us, so we'd prefer to stay with the Carnival excursion list this time. However it looks like there are only 3 excursions for snorkeling on Carnival's site.

• Snorkeling Wonders of the Barrier Reef
  
• Sergeant's Cay Snorkel Adventure
  
• Barrier Reef Snorkel & Starfish Island Getaway
  

Does anyone know which of these might have the best coral and fish for underwater photos? Do any of these require to you to stay with the guide?

Sergeant's Cay Snorkel Adventure seems to be open water with a little island break afterwards. But there aren't any decent photos on the Carnival site. Was the snorkeling good here? I noticed it was .5 hours shorter than the other two excursions, anyone know why? It is considered moderate activity and the other two are easy.

Is the Barrier Reef Snorkel & Starfish Island Getaway actually on a catamaran or trimaran? The photos suggest it might not be. It also seems to be a guided tour only, is that true?

The Snorkeling Wonders of the Barrier Reef seems to be a beginner snorkeling excursion, am I wrong about that?