Map from Google - Krabi

The climate of Krabi is tropical, hot all year round, with a rainy season, due to the southwest monsoon, which runs from May to October, and a dry season from December to March. In March and especially in April, before the monsoon, there is a certain rise in showers, 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.
The town of Krabi, the capital of the province of the same name, is home to an airport, and more than a tourist attraction in itself it is the starting point to beaches, islands and natural parks, such as the beaches of Ao Nang, Phra Nang, Noppharat Thara, Railay, Tonsai, Klong Muang, and the islands of Koh Siboya, Koh Jum, Koh Lanta, Koh Hong, Koh Yao Yai, Koh Yao No, the Phi Phi islands, the Four islands, and parks such as the Ao Phang Nga National Park and the Than Bok Khorani National Park.

Krabi Province, map

Temperatures are high throughout the year, so that on average the maximum temperatures never drop below 30 degrees. However, from December to March the humidity is slightly lower. At night, temperatures rarely drop below 20 degrees, although this may happen sometimes between December and March. From February to April, before the arrival of the monsoon, there is a slight rise in temperature, so this is typically the hottest time of the year (along with May, the first monsoonal month), so much so that it often reaches 35/36 degrees. It goes a bit better in the islands and along the beaches, also because of the sea breeze.
Here are the average temperatures.
Average temperatures - Krabi
Krabi J F M A M J J A S O N D
Min (°C) 22 22 23 24 24 23 23 23 23 22 23 22
Max (°C) 33 34 34 34 33 32 32 31 31 31 31 31

Rainfall amounts to 2,000 millimetres per year; in this coast overlooking the Andaman Sea, 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 hit it directly.
Here is the average precipitation.
Average precipitation - Krabi
Krabi J F M A M J J A S O N D Year
Prec. (mm) 30 45 100 145 170 195 200 265 275 325 180 70 2000
Days 5 5 9 12 16 16 18 18 19 22 18 10 166

The amount of sunshine in Krabi is good in the dry season, from December to March, while during the rainy season the sun does not fail, but it alternates with some cloudiness, which can be more or less durable, and can give rise to rain showers.

From May to November, Krabi like the rest of Thailand can be affected by tropical cyclones, although this area is relatively protected, since cyclones develop in two areas, 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); however, sometimes some of them can pass even over this area.

Phra Nang Beach

For swimming, the sea in Krabi and surrounding areas is warm all year round, as you can see in the following table.
Average sea temperatures - Krabi
Krabi J F M A M J J A S O N D
Sea (°C) 28 28 29 30 30 30 29 29 29 29 29 28

When to go

The best period to go to Krabi for a beach holiday is from December to March, being the dry season. In March and April it is hot and the pre-monsoon rains become more frequent, so the absolute best time is from December to February.