When is a good time to go?

The Perhentian Island are designated as a marine park area and snorkeling is safe for both young and old. Children as well as adults are sure to have great time swimming among the wonders or marine life, including, of course, the local turtles. Diving and snorkeling equipment is available for rent at the Resort.

This is what I've read from this site:
http://www.virtualtourist.com/travel/Asia/Malaysia/Negeri_Terengganu

Basically, the island will start opening end of monsoon season which is early March and will be closed at the beginning of monsoon which is around November. The best time for snorkeling is between April and September. As for diving, the visibility will be at its best between April and August. However, both snorkeling and diving are available from March to September. Most dive center will close early or mid October and snorkeling will not be advisable either due to the fairly rough seas. Peak season will be around July and August when the beach will be most crowded.

So to everyone, plan your holiday well. Welcome to Pulau Perhentian!!!