Map from Google - Martinique

Martinique has a hot and humid climate throughout the year, with limited temperature variations: during the day it goes from 28 degrees Celsius (82 °F) in the "winter" months (December to February), to 30/31 °C (86/88 °F) in the "summer" months (May to October). The relative humidity is above 70% even in the afternoon. During the cooler period, from November to April, the north-east trade winds, steady winds typical of tropical climates, blow regularly.
Here are the average temperatures recorded at the airport.
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, with Fort-de-France as the capital.
The rainfall is abundant: on the coast, 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 on average, so there is not a real dry season. On the slopes exposed to the northeast trade winds, especially those of Mount Pelée, 1,397-metre high, the rains are more abundant, so that the climate can be called equatorial, as evidenced by the lush vegetation.
Here is the average precipitation in the capital.
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

The rains in Martinique occur throughout the year especially as downpour or thunderstorm, hence they do not reduce too much the sunshine hours, except when the island is affected by tropical cyclones, called hurricanes in this region. Hurricanes may occur from June to November, but are most likely from August to October, as in all the Antilles.

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, which is the least rainy period. December and January are still rainy, but less than in the peak period of July-November: considering that they are in the least hot period, and that, however, there may be some showers in all seasons, in the end, you can also choose December and January.

What to pack

All year round: light clothes of natural fabric, 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 climbing Mount Pelée, walking shoes, a light sweatshirt and a light raincoat.