We did a great snorkeling trip through Carnival Valor when we were in Belize. We went to the reef, which is second to the Great Barrier Reef in size, then the took us to a private island that was great. They picked us up at the ship and we were snorkeling in about 30 minutes, then they dropped us back at the ship. We also did a very good snorkel in Roatan through the ship, but this was on Costa. The last time we went to Roatan on Carnival, we just stayed at Mahogany Bay.
The only place I know about a dolphin swim excursion is in Cozumel at Chankanaab Park. I didn't see one listed in Belize or Roatan, but wasn't really looking. We did ours at Chankanaab and it was fabulous. We booked that through the ship as well (on Princess).
Hope this helps a little.