Thailand is either hot and dry (Nov-Apr) high season, or hot and wet (rest of year) low season.
The former as the cool NE continental air floods the country. The latter given moist SW monsoon winds
By far the best months (and most expensive) are Jan and Feb. Cooler weather and beaches are fantastic and water is calm.
Beaches in Phuket are dangerous in summer months and many people drown because the beaches are unsupervised, the sea is very rough with numerous rips.
Summer is by far there cheapest time to visit, but I wouldn’t rush to go back. Not like Jan and Feb when I would go back is a heartbeat.
Been to Phuket, Krabi, Phi Phi and Koh Samui and enjoyed my holidays.
Highlight was renting long tail boat in December and snorkelling around Koh Phi Phi Ley where The Beach was filmed. Although much more to it that the beach.
Thailand is a fantastic winter destination. If you have to go in the summer (with the exception of Koh Samui which has better weather given it is on the east side) give it a miss and go to Bali where the weather patterns are the opposite of Thailand.
Ryan will be along to give more advice.