Map from Google - Saint_Martin

The climate of Saint Martin is tropical, hot and sunny all year round: the average daily temperature goes from 25 °C (77 °F) in the coolest period, from January to March, to 28 °C (82 °F) in the hottest and most humid period, which goes from June to October.
Here are the average temperatures of the capital Marigot.
Average temperatures - Marigot
MarigotJanFebMarAprMayJunJulAugSepOctNovDec
MarigotJFMAMJJASOND
Min (°C)222223232425252526252423
Max (°C)282728282930303131302928
Min (°F)727273737577777779777573
Max (°F)828182828486868888868482

Saint Martin is an island of the Lesser Antilles, divided administratively between France and the Netherlands: here we are dealing with the French part, which is located on the northern side.
Usually, the rainfall in Saint Martin is not very abundant, since it amounts to around 1,000 millimetres (40 inches) per year; February and March are the driest months, with 45/50 mm (1.8/2 in) of rain per month. The rains, which occur in the form of downpour or thunderstorm, are quite irregular, however, they usually increase from August to November.
Here is the average precipitation.
Average precipitation - Marigot
MarigotJanFebMarAprMayJunJulAugSepOctNovDecYear
MarigotJFMAMJJASONDY
Prec. (mm)755045801007085115120100115901045
Prec.(in)321.83.13.92.83.34.54.73.94.53.541.1
Days1087810891213121312122

The amount of sunshine is good all year round, at least along the coasts, as can be seen in the following table.
Sunshine - Marigot
MarigotJanFebMarAprMayJunJulAugSepOctNovDec
MarigotJFMAMJJASOND
Sun (hours)889988898888

In the period from June to November (but more likely from August to October), the island can be hit by tropical storms and hurricanes.

Marigot

In Saint Martin, the sea is warm enough for swimming all year round, although it is a bit warmer in the summer months, as we can see in the following table.
Sea temperature - Saint Martin
Saint MartinJanFebMarAprMayJunJulAugSepOctNovDec
Saint MartinJFMAMJJASOND
Sea (°C)262626262728282829292827
Sea (°F)797979798182828284848281

When to go

The best time to visit the island that includes Saint Martin goes from December to April, and in particular February and March, which are the least rainy months. In May, both temperature and moisture increase, but the rains are not yet too abundant, and the hurricane season has not yet begun. In summer, June and July are less rainy than August and September, and present also a lower risk of hurricanes.

What to pack

All year round: light clothes of natural fabric, possibly a light sweatshirt and a light raincoat for thunderstorms, especially from August to November; a light sweatshirt for the evening from December to March.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.