These are Flamingo Tongue shells on a seafan.  The pictures here are shown at twice life size.      At the very top of the shell directly on the right you can just make out the tube which the mollusk uses to breathe.            The colorful patterns aren't actually on the shell itself.  Instead, these patterns are on the "mantle", part of the soft tissue inside the shell, which the mollusks spread around the shell.  The picture directly below shows what these shells look like without the mantle on them.  Now you can see why they wrap the mantle around themselves!

A sponge garden, with several different varieties of sponge.  Spot the fish.  Or maybe that should be "lots of spots" the fish.

Depending on who you believe, this is either: (a) a reticulated sea star;  (b) a reticulated starfish;  (c) a cushion star or  (d) a Bahama starfish.  And if you speak Latin, then it's an Oreaster reticulatus.

The photo above is of a Feather Duster Worm.  When threatened, they retreat into the hard tube which they construct.  Apparently there are plenty of nudibranches in the Caribbean, but I've seen far more in the Pacific.  On the left is a nudibranch, about twice life size, called Elysia crispata, which is extraordinarily variable in color and appearance.   The frills are used to house photosynthesizing cells which it extracts from the algae it eats.

 

Anemones, looking just like anemones in the Pacific.  But if this were the Pacific, there would also be a clown fish or anemonefish - but there aren't any of these fish in the Carribean.

Close-up of some brain coral.        The biggest regret from my trip to Belize is that I didn't do a night-time snorkelling trip.  Coral polyps only feed at night, so that's the only time to see them.  Night trips were being offered, but like all the other trips there was a minimum of two people.  It never occurred to me that I could pay double the normal single price, and do the trip.  It would have been worth it.