You might get lucky and see a whaleshark but bear in mind they are a treat and some instructors and divemasters living and diving every day on Utila have not seen one !
Here are our whaleshark resources
http://www.whalesharkproject.org/
http://www.bbc.co.uk/nature/life/Whale_shark
Of course wikipedia think it knows it all ! http://en.wikipedia.org/wiki/Whale_shark