Map from Google - Phi_Phi_Islands

The climate of the Phi Phi islands, an archipelago in the Andaman Sea belonging to the province of Krabi, is tropical, hot all year round, with a rainy season from May to October, due to the southwest monsoon, and a dry season from December to March. In April, before the monsoon, there is a certain increase in rainshowers, while in November there can still be abundant rainfall, especially in the first part, either for any delay in the retreat of the monsoon, or by the passage of a tropical cyclone (see below). However, there may be some showers or downpours also in the dry season, especially in December, when occasionally one among them can be particularly intense.
The archipelago is made up of the main islands, Phi Phi Don and Phi Phi Leh, and some smaller islands such as Ko Mai Phai (also called Bamboo Island), Bida Nok and Bida Nai. They are coral islands, where tourists scuba dive. The islands have been hard-hit by the tsunami in December 2004, and the infrastructures have been rebuilt in the following years.

Map of Phi Phi islands and surrounding areas

Temperatures are high throughout the year, in fact average highs never drop below 30 °C (86 °F). However, from December to March the humidity is slightly lower. Night temperatures very rarely drop below 20 °C (68 °F), although this may sometimes happen between December and March. From February to April, before the arrival of the monsoon, there is a slight rise in temperature, so March and April (along with May, the first monsoon month) are usually the hottest months of the year, in which highs can quite often reach 35/36 °C (95/97 °F).
Here are the average temperatures of the main island, Phi Phi Don.
Average temperatures - Phi Phi Don
Phi Phi DonJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)222223242424242424232323
Max (°C)313333333332313131313031
Min (°F)727273757575757575737373
Max (°F)889191919190888888888688

In the Phi Phi islands, an average of approximately 2,400 millimetres (94.5 inches) of rain falls each year; in these islands and in the Andaman Sea coast, the rains during the period of the southwest monsoon are more abundant than in the other coast of peninsular Thailand, that of the Gulf of Siam, because the south-western winds affect them directly. However, the rainy season sees two peaks, in the first part (May) and the last (September-October). During some years, however, there can be torrential rains also in July and August.
Here is the average precipitation.
Average precipitation - Phi Phi Don
Phi Phi DonJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)502550130310260275260375330230902380
Prec. (in)2125.

The amount of sunshine in the Phi Phi islands is good in the driest period, from late December to March, while during the rainy season the sun is not missing, but it alternates with cloudiness, which can sometimes give rise to rain showers.
Here are the average daily sunshine hours in nearby Krabi.
Sunshine - Krabi
Sun (hours)686655555456

From May to November, the Phi Phi islands like the rest of Thailand may be affected by tropical cyclones, although this area is relatively protected, since cyclones develop in two areas, one to the west (the Bay 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); sometimes however one of them can pass here as well.

The Beach, Maya Bay, Phi Phi Leh island

For swimming, the sea in the Phi Phi island is warm all year round. It should be said that in summer the sea can be rough due to the monsoon, and there can be dangerous currents.
Here is the average sea temperature.
Sea temperature - Phi Phi Don
Phi Phi DonJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)282829303030292929292928
Sea (°F)828284868686848484848482

When to go

The best time to go to the Phi Phi islands and enjoy the sun and the sea goes from late December to March, being the dry season. As mentioned, in December and early January at times there may still be some intense showers, while in February and March it can be hot.