The most popular question Koh Samui dive instructors get asked is: "When are conditions good for diving around Koh Samui?" Here are some answers:

March to April is famous for great conditions -25-40 meter visibility  - and there are also great conditions from July to November.

December to February can be tricky as some years, Koh Samui can get a lot of rain and weather, so conditions aren't so good, but other years, with less rain (every few days for an only hour or so) conditions are much improved.

In 2005, the rain and storms came late and Samui diving shops did not run much diving from the end of November to the end of December.

In 2007, Samui had poor visibility, less than 10 meters, from mid July to mid December7/11/2007 until 12/11/2007 which improved to an OK visibility (on average 18 meters).

Please note that when Koh Samui diving is canceled due to big storms (on average 10 days a year), the west cost diving is also cancelled as the big storms effect both sides of Thailand's coast. 

Unfortunately, in Koh Samui most big dive companies will run trips and will advise you of great visibility regardless of condition.  Some companies will cancel trips if visibility and conditions are not good, so you need to do some research.