CAMBODIA
When people think of diving in southeast Asia, most overlook Cambodia as a destination. Of course countries like Thailand, Indonesia, and the Philippines take the front seat, but as you travel around chances are you will end up in 
Cambodia if only to see Angkor Wat. While you are there, I see absolutely no reason why you shouldn’t take advantage of the opportunity Sihanoukville offers divers.
Located in southern Cambodia with diving in the Gulf of Thailand, it’s not a surprise the diving is similar to other destinations in the Gulf. The first PADI dive center in Cambodia opened in 2002, so diving here is fairly new. Most dive operations offer overnight trips including 5 or 6 dives for a very good price. The diving is relaxed and pleasant, without the congestion you find on for example, Koh Tao in Thailand. The islands off the coast of Cambodia provide excellent diving and white sand beaches. There is some great diving close to the mainland, but for those who want to stay a little longer and venture a little further, there are world class dive sites such as Condor Reef and Poulo Wai waiting to be explored. The diving season is from late October to May/June and visibility typically ranges from 10-25+ metres. Obviously one of the main attractions to diving here is that it has been dived so little that most of the dive sites are unmapped and lots of the diving is still at exploration stage.
The town of Sihanoukville is a great beach-side town offering four beaches and 
many resorts to choose from. Upon entering Cambodia tourists are granted a 15 day visa. You only need three days to see Angkor, so why not hop down to Sihanoukville and jump in the water to cool off. Afterwards, a stopat the riverside town of Kampot is worthwhile. It is nearby and offers a national park with waterfalls, hiking trails and relaxation.