Close to the Cambodian border in the Gulf of Thailand you find Koh Chang, quite a large island surrounded by blue ocean. Located about 6 hours from Bangkok by bus, Its closest airport is located in Trat however flights are still quite steep. While the diving in the area isn't as impressive as you might find elsewhere it does have one of the best recreational dive wrecks in the nation, the HMS Chang. Close by you also find Koh Kood, a even more chilled out island with its own unique scuba dive sites.
Similar to elsewhere in the Gulf of Thailand, Koh Chang and Koh Kood experience monsoon season between December and January and has its best dive season between March and May. However you can dive here all year due to the protection the islands provide during the seasons.
You can expect healthy reefs and inhabitants such as turtles, sea snakes and barracuda. Whale sharks do visit the islands yearly but not as frequent as in other areas. If you like macro life, Koh Kood has quite a healthy sea horse population!
The highlight dive here is the stunning wreck HMS Chang, sunk in 2012. The ship was formerly known as the USS Lincoln Country and served in the pacific theater during WWII and in the Korean War. Despite its few years underwater it has attracted huge amounts of marine life, making it an absolute must if you visit Koh Chang.