Map from Google - Nosy_Be

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. The island is located off the northwest coast of Madagascar, in an area that in the austral summer months 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 millimetres of rain fall. In the rainiest months (December, January and February), the rains are very frequent, occurring almost daily, and can sometimes be torrential. On the contrary, at the height of the dry season, from June to September, showers are very rare; in May and October they are a bit more frequent, but generally not intense.
Here is the average precipitation.
Average precipitation - Nosy Be
Nosy BeJanFebMarAprMayJunJulAugSepOctNovDecYear
Nosy BeJFMAMJJASONDY
Prec. (mm)4654252851406050354040951903602185
Days20161373333351014100

In the period that could be called winter, from June to September, at night the temperature can drop below 20 °C, and sometimes can even around 15/16 °C, 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.
Average temperatures - Nosy Be
Nosy BeJanFebMarAprMayJunJulAugSepOctNovDec
Nosy BeJFMAMJJASOND
Min (°C)232323222019181819202223
Max (°C)313131313129292931323231

The amount of sunshine in Nosy Be is good from May to October, while it is acceptable in the rainiest months, which as we said are December, January and February.

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

Nosy Be

For swimming, the sea is warm all year round, since the water temperature ranges from 26 °C in the period from July to October, to 29 °C from January to April.
Here are the average sea temperatures.
Sea temperature - Nosy Be
Nosy BeJanFebMarAprMayJunJulAugSepOctNovDec
Nosy BeJFMAMJJASOND
Sea (°C)292929292827262626262828

When to go

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 the passage of tropical cyclones, as happened with cyclone Kesiny, which passed over this area around the 10th of May 2011.