As a certified diver (Are you?.. I guessed you are because you asked for diving not getting diving lessons) you usually know to find the information about dive sites in www.padi.com website firstly. Then you can search more by giving some key words to the search engines and complete it.
Dive Season: Year-round. Whale sharks sightings are virtually guaranteed from April to June along the southern barrier reef.
Dive Season: All year
There is not a best time for diving or in the other word it depends on your purpose, the kind of accommodations which you expect, the kind of species and sea life you want to see under the water and surely not all species are visible whole around the year, your free time and the weather condition that you feel more comfortable to dive. You can find all them in those two pages and select your best time according to your own favorite imagination of a favorite dive time.