Big Blue Planet knows about the following countries in this region:

Indonesia is the world's largest Archipelago with over 17,000 islands. Well trodden destinations such as Bali and Java are easily accessible, but others such as Sulawesi and Sumatra are definately for the more adventurous. Waters are very nutrient rich making the diving stunning with fantastic corals and macro photography opportunities.
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Malaysia is a federation of 13 states in southeast asia split physically into the Peninsula Malaysia, and the island of Borneo. Easy to get to with a good tourist infrastructure it contains the famous Sipadan islands as well as the lesser know Layang Layang corral reefs. On the Malysian Peninsula you can also find some well known diving locations such as Perhentian Besar.
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The second largest island archipelago in the world behind Indonesia, with a hot, humid, and tropical climate, the Philippines has suffered greatly from Dynamite and Cyanide fishing practices. However, a lot of the reefs have regenerated and are now exceptional. Also home to 8 wrecks in Coran.
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A beautiful country with white sand beaches, green forests, and very friendly people, it is also a well known tourist destination with over 300,000 divers a year. Most diving takes place on both sides of the peninsula with the best being the Similan islands. Other dive sites (particularly Phuket) will be very crowded and commercialised.
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