No typhoon has ever struck the tiny island of Koh Samui. Well, at least not within the living memory of the islanders. That is not to say a typhoon could never hit, but let's put it this way; the elderly islanders never seem to waste time talking or worrying about typhoons that have the power to take human life because such tragedies are simply not within their personal experiences.

The fishermen of course, know all about bad weather because its their job to know and as far as they are concerned Koh Samui is considered a safe haven and always has been.

But, when a typhoon is forming somewhere in the many seas of South-East Asia - the island is bound to get some peripheral rainfall and strong squalls and a few old trees might topple but this would be the expected affect of any storm anywhere on the planet.

Koh Samui does have a few small rivers, or rather streams, but even in a sustained rainfall these are too tiny to burst their banks and cause havoc to the locals. Not like the mainland where flash flooding is a common occurance and quite life-threatening. 

However, if you want to avoid even the local storms caused by a distant typhoon, then it's best to avoid the monsoon season altogether - which is between November and December. That leaves you ten other months when the weather is balmy and calm.

The weather system in Thailand is influenced by two monsoons: the northeast monsoon and the southwest monsoon. The SW monsoon affects the Andaman Sea (western-side) of Thailand in June / October.

The NE monsoon affects the eastern-seaboard of the Gulf of Thailand (where Koh Samui is located) and during November and early December the seas can tend to be rough if a typhoon is being created somewhere else in South-East Asia. That's not the best time to visit if you want to go coral or wreck diving.

This is not to assume that Koh Samui does not get flooded now and again (especially late October - early November) as this link will testify. But we are still not talking life-threatening floods - we are talking about very inconvenient floods - and then mainly in the low-lying areas.