Nosy Be, where it's located

In Nosy Be (or Nossi-bé), the climate is tropical, hot all year round, with a rainy and hotter season from November to April, and a dry season from May to October.
In the period that could be called winter, from June to September, at night the temperature usually drops below 20 °C (68 °F), and sometimes it can even drop to around 15/16 °C (59/61 °F), but during the day it's almost always warm, thanks to the winds that blow from the interior of Madagascar.
Here are the average temperatures.
Nosy Be - Average temperatures
Min (°C)232323222019181819202223
Max (°C)313131313129292931323231
Min (°F)737373726866646466687273
Max (°F)888888888884848488909088

The island is located off the northwest coast of Madagascar, in an area that in the austral summer is affected by the hot and humid monsoon coming from the north-west, while in the austral winter the cooler and drier south-east trade winds prevail. The climate of the neighbouring islands, such as Nosy Komba, Nosy Mitsio, Nosy Tanikely and Nosy Sakatia, is similar.

In an average year, about 2,200 millimeters (86 inches) of rain fall. In the rainiest months (December, January and February), the rains are definitely frequent, in fact they occur almost daily, and sometimes they can be torrential. On the contrary, at the height of the dry season, from June to September, there are rare showers, which in May and October are a bit more frequent.
Here is the average precipitation.
Nosy Be - Average precipitation

The amount of sunshine in Nosy Be is good from May to October, while it is acceptable in the rainiest months, from December to February, when, apart from showers and thunderstorms, the sun shines for a few hours a day.
Nosy Be - Sunshine

From November to mid-May, Nosy Be may be affected by tropical cyclones of the South-West Indian Ocean, which in any case are more common from January to March.

Nosy Be

The sea in Nosy Be is warm enough to swim in all year round, since the water temperature ranges from 26 °C (79 °F) in the July-October period to 29 °C (84 °F) between January and April. Here are the average sea temperatures.
Nosy Be - Sea temperature
Temp (°C)292929292827262626262828
Temp (°F)848484848281797979798282

Best Time

The best time to go to Nosy Be goes from mid-May to October. In the first half of May, usually the weather is already good, but there is still some risk, though low, of tropical cyclones, as happened with cyclone Kesiny, which affected this area around May 10, 2002.