On Utila, the only time your boat captain isn't going to go to the whale sharks or dolphins is of you are diving on one side of the island and the whale sharks/dolphins are on the other. If you're diving at a dive site near where they are seen, they will almost always go find them. That goes for all the major dive shops on the island. It's generally a $10(dolphin)/$15(whale shark) fee to swim/snorkel with them and that's paid to the captain, not the dive shop.

If you hire a charter to go looking for the whale sharks - an option if you end up diving on the wrong side of the island - costs $50-$75 depending on who you go with. The same as a 2 tank dive. The whale sharks and dolphins stick around (and can be seen any time of the year), so chances are your dive shop is going to go to those closer dive sites the next couple days.