there arent really any whale sharks in the waters up here, but after you have exhausted all the fun here in belize, you can head to honduras and check them out there (during season, although you can get lucky other times of the year from what i understand). I can refer you to some dive shops there if you are interested.

Someone else chime in here, but i do think that sometimes you can see them out of PG, but i'm not sure about that.