Not sure how the snorkeling trips work for whale shark spotting but I would recommend diving rather than snorkeling, it is unlike any other diving you will do.
Basically it is a 2 hour or so boat trip from Placencia out to Gladden Split.
You then wait for your chance to encounter the whale sharks, they limit the amounts of boats (and divers, normally about 8 divers per boat) allowed into the split at any one time in order to protect the creatures.
So you go into the Split and after the captain of the boat has spoken to the local fisherman to see where the snappers are spawning (the whale sharks come to the Split to feed on the snapper spawn).
Then you all get into the water (very quickly) descend to about 60 feet and stay together as a group while the Dive master try to spot the balls of snapper.
You are in total open water there is no coral or anything else aprt from blue water.
So you are then swimming around really fast following the DM until the DM sees something.
You then get together as a really tight group making as many bubbles as possible (concept being that the Whale sharks think you look like a ball of spawning snapper)
You are then looking around like a headless chicken, banging tanks with the other divers waiting for something....then, if you're lucky, the fun starts!! They arrive and to see these magnificent 20-30 feet creatures up close will live in your memory for ever.
We were so lucky we had 2 dives and saw 4 on the first dive and 2 on the second.
As for where to stay it really depends on your budget, the Inn at Roberts Grove is really nice but expensive, but there are plenty of inexpensive places to stay. JUST BOOK EARLY since a lot of divers want to see the whale sharks.