If you want to enjoy great weather in Koh Lanta, the best time to go is November to May when the possibility of rain is low. The down side is that December and January is peak tourist season which means hotel accommodation cost more. Prices taper off in February and March, but April and May when weather is actually at its best falls within the low season when you can expect bargain hotel deals.
But even if you visit the island during rainy season, June until end October, we are not exactly talking European winters. Unless you are here for the exclusive purpose of scuba diving and can handle a few showers every now and again it is still a good time to visit. Temperatures on Koh Lanta is fairly stable all year round with maximum temperatures averaging 30 to 34 degrees Celsius and minimum temperatures averaging 20 to 25 degrees Celsius. Water temperatures can be expected to be around 28 degrees.
The climate is tropical and predicting the weather in tropical conditions is always difficult. You may visit Lanta island for a week in the rainy season and only see sunny skies, while you may have a shower or two during the dry season. Many people do in fact like the cooling effect of a few showers to freshen things up.