Map from Google - Mayotte

The island of Mayotte, overseas department of France, is located on the Indian Ocean, at a latitude of nearly 13 degrees south. Its climate is tropical, with a warm, wet and rainy season from December to April, when the north-west monsoon prevails, and a cool, dry and sunny season from June to October, when the south-east trade winds prevail, while May and November can be considered as transitional months.
The average daily temperature ranges between 81 °F in the warmer period (January-April), to 73/75 °F in the least warm period (June-September). From June to September, the temperature never exceeds 90 °F, while from November to March, it can sometimes reach 93/95 °F, but in this period it's the humidity which makes the air uncomfortable in this period.
Here are the average temperatures.
Average temperatures - Mayotte
MayotteJanFebMarAprMayJunJulAugSepOctNovDec
MayotteJFMAMJJASOND
Min (°F)757575757372707070727375
Max (°F)868686868281797982848888

The total annual rainfall is not very high, and amounts to 48 inches in the small island of Pamanzi, and 56 inches in the main island, most of which occur from October to May; the rainiest month is January, when the rains are heavy.
Here is the average precipitation.
Average precipitation - Mayotte
MayotteJanFebMarAprMayJunJulAugSepOctNovDecYear
MayotteJFMAMJJASONDY
Prec. (in)12.29.67.54.92.81.41.61.21.22.63.57.356.1
Days19161611654446915115

During the rainy period, however, the amount of sunshine is acceptable, because the tropical rains occur mainly in the form of downpour or thunderstorm, intense but not long lasting. Anyway, this period, in addition to being sultry, also presents the risk of tropical storms or cyclones, which are more likely from late December to mid-April.
During the dry period, some brief thunderstorms are still possible, but in the island of Mayotte they are very rare in the period of the trade winds, because it is flat (therefore there are no mountain ranges which could be able to foster cloud formation), and also because it is protected by Madagascar, which is located just 185 miles to the south-east.

Mayotte

The sea temperature is always warm enough for swimming, in fact it fluctuates from 79 °F from July to September, to 84 °F in the February-April period.
Sea temperature - Mayotte
MayotteJanFebMarAprMayJunJulAugSepOctNovDec
MayotteJFMAMJJASOND
Sea (°F)848484848281797979818282

When to go

The best time to visit Mayotte for a beach vacation goes from June to October: the temperature is pleasantly warm, the sunshine amount is good and the rains are rare.

What to pack

In winter (June to August): light clothes, a sweatshirt for the evening, a scarf for the breeze. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In summer (December to February): tropics-friendly, loose fitting clothing, light shirts and pants of natural fibers (cotton, linen), light sweatshirt for the evening, light raincoat or umbrella for the rain showers.