Map from Google - Mayotte

The climate of Mayotte is tropical, with a hot, humid and rainy season from December to April, when the north-west monsoon prevails, and a relatively 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.
Mayotte is an archipelago consisting of two main islands, Grande-Terre (or Maore), and Petite-Terre (or Pamanzi), plus some smaller islets. The archipelago is an overseas department of France, and is located in the Indian Ocean, at a latitude of nearly 13 degrees south.
The average daily temperature ranges between 27 °C (81 °F) in the warmest period (January-April), to 23/24 °C (73/75 °F) in the least warm period (June-September). From June to September, the temperature never exceeds 32 °C (90 °F), while from November to March it can sometimes reach 34/35 °C (93/95 °F), morevoer, high humidity in this period makes the heat uncomfortable.
Here are the average temperatures in the main island.
Average temperatures - Mayotte
MayotteJanFebMarAprMayJunJulAugSepOctNovDec
MayotteJFMAMJJASOND
Min (°C)242424242322212121222324
Max (°C)303030302827262628293131
Min (°F)757575757372707070727375
Max (°F)868686868281797982848888

The total annual rainfall is not very high, and amounts to 1,200 millimetres (47 inches) in the small island of Pamanzi, and 1,400 mm (55 in) in the main island, most of which occur from October to May; the rainiest month is January, when the rains are definitely abundant.
Here is the average precipitation.
Average precipitation - Mayotte
MayotteJanFebMarAprMayJunJulAugSepOctNovDecYear
MayotteJFMAMJJASONDY
Prec. (mm)310245190125703540303065901851425
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 downpours or thunderstorms, which can be intense but do not last long. This period, in addition to being sultry, also presents the risk of tropical storms or cyclones, which, however, are more likely from late December to mid-April.
During the dry period, some brief thunderstorms are still possible, especially in the main island, where there are some small elevations: the highest point is Mount Benara, 600 meters (2,170 feet) high; in addition, the archipelago is protected by Madagascar, which is located at just 300 kilometres (185 miles) to the south-east.

Mayotte

The temperature of the sea in Mayotte is always warm enough for swimming, in fact it fluctuates from 26 °C (79 °F) from July to September, to 29 °C (84 °F) in the February-April period.
Sea temperature - Mayotte
MayotteJanFebMarAprMayJunJulAugSepOctNovDec
MayotteJFMAMJJASOND
Sea (°C)292929292827262626272828
Sea (°F)848484848281797979818282

When to go

The best time to visit Mayotte for a beach holiday 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 fibres (cotton, linen), light sweatshirt for the evening, light raincoat or umbrella for the rain showers.