Samui offers high-quality diving most of the year, though it's probably best avoided during the monsoon in November and December. It is still possible to take shore-based courses during this time however. The four-day PADI Open Water Course is very competitively priced compared to elsewhere in the world and there's a wide range of schools to choose from. The most famous dive site in the area is Sail Rock, North of Ko Pha Ngan, which has an 18m natural rock chimney swimthrough and is a good place to see whale sharks. Other sites can be found in the North of Anthong Marine National Park and close to the Gulf's diving mecca Ko Tao, which has 21 sites off its shores. A day's diving from Samui should get to two to three sites.