Peak whale shark diving is during the period 4-5 days before and after the full moon the dates for April are
APRIL 7th - 19th
Spend a couple of days in Placencia and go with Brian Young at Seahorse Dive Shop. Make sure you book April trip get booked very early
Here is his website with all info about Whale Shark Diving. If the whale sharks are out thee they will find them
http://www.belizescuba.com/Here is a link to some of the pictures I took a few years ago
//ambergriscaye.com/forum/ubbthreads.php/topics/21388/Re_Seahorse_Dive_Shop.html#Post21388
Colin