J don't count on seing whale sharks on the south side of Roatan. or big pelagics for that matter. That's not their habitat. We are talking about relatively shallow warm water here. Great for juveniles of all sorts and small macro critters! If you do see one, buy yourself a lotery ticket!
If you want chances of whale sharks, you need to head to Utila.
Whales is another thing. While I was there we spotted some pilot whales. Unfortunately they were headed in the other direction so we were unable to snorkel with them. But it could have happened!
Cheers!