In Peninsular Thailand, the climate is hot all year round.
The heating between March and May is less intense than in the continental region, and on average these months are slightly warmer than in the rest of the year: highs are around 33/34 °C (91/93 °F) instead of 31/32 °C (88/90 °F), although sometimes it can get very hot here too. Additionally, in the peninsular area, the rainy season occurs at different times depending on the slope.
Let's see when. Along the east coast, on the Gulf of Thailand (or Gulf of Siam), from Chumphon to the south, the summer monsoon is not felt in a particularly intense way, so much so that summer is considered high season, and in this period the area is frequented by many tourists.
On the contrary, the so-called retreating monsoon, in the early months (October to December), when the sea is still warm and able to provide energy and moisture, brings heavy rainfall, sometimes torrential.
Sometimes the rains continue until the beginning or the middle of January, so in this area, which includes Ko Samui, Pattani and Songkhla, the best time is from the middle or the end of January to April (with a preference for February, since even here March and April are a bit hot).
Here are the average temperatures in Ko Samui. Despite the different rainfall pattern, the sunshine has a relative minimum in the period June-October on the east coast as well, although it remains quite low even from November to January.
In Ko Samui, and in general in Thailand, the sea is warm all year round, as you can see from the water temperature near Ko Samui.
Along the west coast, on the Andaman Sea (see the Similan Islands, Phuket, Krabi, the Phi Phi Islands, Koh Lanta, Koh Lipe), summer is very rainy, since the monsoon hits the coast directly from the sea, while there's little rain from December to March.
In addition, during the summer monsoon the sea can be rough, and reaching the islands by boat can be difficult.
The amount of sunshine on the west coast is quite limited in the May-October period. So here the best time is from late December to March; in April, pre-monsoon showers are more frequent than elsewhere. Here are the average temperatures in Phuket.