The whalesharks migrate to Gladden Spit (or 'Split' depending on who you talk to!) between March and July or August. And are seen during the full moon when the snapper are spawning!! As snorkelers you will have a very good chance to see them. Whalesharks are very docile and shy. They typically hang out very deep and then swim upward toward the surface feeding on the snapper spawn. Their only interest is in the spawn and very much ignore the human spectators! (unless the humans make that impossible) Their swim is very slow, quiet and methodical. As a diver, you can be deeper than a snorkeler and look one way and maybe miss them. I can say from experience, though, it is an awesome vision!! I will say it is worth the trip to Placencia!!
Hope you have a great time!!