Map from Google - Martinique

In Martinique, the climate is tropical, hot and humid all year round, with a relatively cool and dry season from January to mid-April, called carême (Lent), and a hot, humid and rainy season from mid-June to mid-November, called hivernage (whose root reminds of the winter, although it is a hot season, and also quite sunny). In the former period, the northeast trade winds, constant winds typical of tropical climates, blow steadily and with moderate intensity, while in the latter period the winds are more irregular, and may have some breaks, increasing the feeling of sultriness. Between them there are two transitional periods: from mid November to early January (when the northeast wind starts to blow, the temperature decreases a bit and the weather gets better), and from mid-April to Mid-June (when both the temperature and the frequency of downpours gradually increase).
The relative humidity is high throughout the year, and is above 70% even in the afternoon.
Here are the average temperatures recorded in Le Lamentin, the second city of the island. As you can see, there are little variations between the coolest and the warmest period.
Average temperatures - Le Lamentin
Le LamentinJanFebMarAprMayJunJulAugSepOctNovDec
Le LamentinJFMAMJJASOND
Min (°C)212122222324242423232322
Max (°C)282829303030303131313029
Min (°F)707072727375757573737372
Max (°F)828284868686868888888684

Martinique is an island, which has the status of a French overseas department.
Rainfall in Martinique varies greatly in quantity depending on the area, but follows the same pattern, and are more frequent and abundant from May to November (and especially from July to November), and less frequent and shorter from December to April (and particularly in February and March).
The northern part of the island, occupied by hills and mountains, is rainier than the south, in particular on the slopes of Mount Pelée, 1,397 metres (4,583 feet) high, and of the Carbet Mountains, which reach a height of 1,197 metres (3,927 feet), the rains are very abundant, as they exceed 4,000 mm (160 in) per year, moreover the sky is often cloudy, and there is no real dry season, so much so that the climate can be called equatorial, as evidenced by the lush vegetation.
In the capital Fort-de-France, which is located in the central part of the island, 2,000 millimetres (79 inches) of rain per year fall, of which more than 100 millimetres (4 inches) per month from May to December, and among these, more than 200 mm (8 in) from July to November, which is the rainiest period. The least rainy period goes from February to April, with 65/90 mm (2.5/3.5 in) on average. Here is the average precipitation.
Average precipitation - Fort-de-France
Fort-de-FranceJanFebMarAprMayJunJulAugSepOctNovDecYear
Fort-de-FranceJFMAMJJASONDY
Prec. (mm)1058065901301802552302552252051351955
Prec.(in)4.13.12.63.55.17.1109.1108.98.15.377
Days171213131518222119181817203

In the southern part of the island, precipitation drops below 2,000 mm (79 in) per year, and along the east and south coasts it goes down to around 1,200/1,500 mm (47/60 in) per year.
The rains in Martinique occur throughout the year especially as downpours or thunderstorms, maybe intense, but which generally do not last long, and therefore do not reduce too much the sunshine hours, though sometimes there may be a more intense wave of bad weather.
In fact, the amount of sunshine is good all year round, at least along the coasts, as can be seen in the following table.
Sunshine - Fort-de-France
Fort-de-FranceJanFebMarAprMayJunJulAugSepOctNovDec
Fort-de-FranceJFMAMJJASOND
Sun (hours)888887887778

The greatest danger comes from and tropical storms and cyclones, called hurricanes in the Caribbean area, which may pass from June to November, but are most likely from August to October. A very violent hurricane that hit Martinique was Hugo, on September 16, 1989.

Martinique

The sea is pleasantly warm all year round, as we can see from the following table.
Sea temperature - Martinique
MartiniqueJanFebMarAprMayJunJulAugSepOctNovDec
MartiniqueJFMAMJJASOND
Sea (°C)272626272828282829292827
Sea (°F)817979818282828284848281

When to go

The best time to visit Martinique goes from February to April, being the least rainy period. December and January are a bit more rainy, but less than in the peak period of July-November: considering that they are in the least hot period, and that there may be some showers in all seasons, after all, you can also choose December and January.

What to pack

All year round: light clothes of natural fabric, sun hat, sunscreen, possibly a light sweatshirt and a light raincoat for thunderstorms. A light sweatshirt for the evening from December to March.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
For hiking in mountain ranges, sweatshirt, raincoat and hiking boots.