On the Andaman Islands, located between the Bay of Bengal and the Andaman Sea, at about 10/13 degrees north latitude, the climate is tropical, hot all year round, with a dry season from January to April, and a rainy season due to the Indian monsoon, from May to November.
The temperatures are stable throughout the year, however, before the rainy season, in March and April, the hottest time of the year occurs. Here are the average temperatures in Port Blair, the capital, located on the island of South Andaman.
Port Blair - Average temperatures
Min (°C)232323252524242424242424
Max (°C)293032333130292929303029
Min (°F)737373777775757575757575
Max (°F)848690918886848484868684

Precipitation is abundant, and amounts to almost 3,000 millimeters (120 inches) per year; they begin in May, and are very frequent. The rains decrease in November, and in December they are already quite rare. Here is the average precipitation.
Port Blair - Average precipitation

From December to April the sun regularly shines, while from June to September the sky is often cloudy.
Port Blair - Sunshine

The islands lie in the path of tropical cyclones; they are formed more easily between April and December, with two peaks at the beginning and at the end of the period (April-June and October-December). However, given the warm sea, they can in theory form all year round.

Havelock Island

The sea is warm enough to swim in all year round, as can be seen in the following table.
Andaman - Sea temperature
Temp (°C)282829303029292828292928
Temp (°F)828284868684848282848482

When to go

The best time for a beach holiday in the Andamans is from January to April, since it is the least rainy of the year. Sometimes the rains last until mid-January, while in March and April it can be very hot; moreover, in April the first cyclones can affect the islands, therefore the best period is from mid-January to the end of February.

What to pack

All year round, bring light clothes, of natural fibres, a sun hat, a scarf for the breeze, a light sweatshirt for the evening. In the rainy season, you can add a light raincoat or umbrella.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.