Map from Google - Phuket

The climate of Phuket is tropical, hot all year round, with a rainy season, due to the southwest monsoon, which lasts from May to October, and a dry season from December to March. In April, before the monsoon, there is an increase in rainfall, while in November there may still be abundant rainfall, especially in the first part, either for any delay in the retreat of the monsoon, or for the passage of a tropical cyclone (see below). Even in the dry season, however, there may be some showers, especially in December, when occasionally some of them may be particularly intense.

In an average year, about 87 inches of rain fall; in the coast on the Andaman Sea, where Phuket is located, the rains during the southwest monsoon are more abundant than the other coast of peninsular Thailand, that on the Gulf of Siam, because the south-western winds hit it directly. However, the rainy period sees two peaks, in the first part (in May) and in the last (September-October). In some years, however, there may be torrential rains even in July and August.
Here is the average precipitation.
Average precipitation - Phuket
Prec. (in)1.41.634.911.610.48.59.612.812.47.73.187

The temperature is high throughout the year, so that average highs never drop below 85 °F. However, from December to March, air humidity is slightly lower. Whatever the season, particularly hot days are always possible, with peaks around 97/100 °F, while nigths a bit cool, below 70 °F, can occur from December to March. From February to April, before the arrival of the monsoon, there is a slight increase in temperature, so that March and April are usually the hottest months of the year.
Here are the average temperatures.
Average temperatures - Phuket
Min (°F)737375777777777575757575
Max (°F)889091918888888886888888

The amount of sunshine in Phuket is good in the dry season, from December to March, while during the rainy season the sun shines often enough, but it alternates with some cloudiness, which can sometimes lead to rain showers.

Phuket is the largest Thai island, and is a province along with the small islands that surround it; there are, in addition to the eponymous capital, towns like Patong, Karon, Rawai and Mai Khao. The most famous beaches are Patong Beach, Karon Beach, Kata Beach, Kata Noi Beach, Nai Han Beach, Kamala Beach, Surin Beach and Bang Tao Beach.

Map of Phuket and surrounding areas

From May to November, Phuket like the rest of Thailand can be affected by tropical cyclones, although this area is relatively protected, since cyclones develop in two zones, one to the west (the Gulf of Bengal, where they are called cyclones), and the other to the east (the Pacific Ocean and the South China Sea, where they are known as typhoons), although sometimes some of them can pass even over this area.

Sea in Phuket
For swimming, the sea is warm all year round, as you can see in the following table.
Sea temperature - Phuket
Sea (°F)828284868686848484848482

When to go

The best time to go to Phuket for beach life goes from December to March, being the dry season. However, surfers prefer the period from June to September, when waves are higher in beaches exposed to the west, like Pansea Beach, Karon Beach and Kata Beach.