Map from Google - Sint_Maarten

The climate in Sint Maarten is tropical, hot and sunny all year round: the daily average temperature goes from 25 degrees Celsius (77 °F) in the period from January to March, to 28 °C (82 °F) from June to October. The night temperature rarely drops below 20 °C (68 °F), while sometimes it can touch 35/37 °C (95/99 °F) during the day, from June to November. In the hottest period there's a muggy heat, though the breeze relieves it a little.
Here are the average temperatures of the capital Philipsburg.
Average temperatures - Philipsburg
PhilipsburgJanFebMarAprMayJunJulAugSepOctNovDec
PhilipsburgJFMAMJJASOND
Min (°C)222223232425252526252423
Max (°C)282728282930303131302928
Min (°F)727273737577777779777573
Max (°F)828182828486868888868482

Sint Maarten is an island of the Lesser Antilles, located at a latitude of 18 ° north, and is administratively divided between France and the Netherlands; here we are dealing with the Dutch side, located in the south.
Normally the rainfall is not abundant, since the annual average amounts to around 1,000 millimetres (40 inches); February and March are the least rainy months, with about 45/50 mm (1.8/2 in) per month, while the rainiest period runs from August to November.
Here is the average precipitation.
Average precipitation - Philipsburg
PhilipsburgJanFebMarAprMayJunJulAugSepOctNovDecYear
PhilipsburgJFMAMJJASONDY
Prec. (mm)755045801007085115120100115901045
Prec.(in)321.83.13.92.83.34.54.73.94.53.541.1
Days1087810891213121312122

However, the rains come in the form of downpour or thunderstorm, which do not last long, so that the sun can shine throughout the year, as can be seen in the following table.
Sunshine - Philipsburg
PhilipsburgJanFebMarAprMayJunJulAugSepOctNovDec
PhilipsburgJFMAMJJASOND
Sun (hours)889988898888

In the period from June to November (but mostly from August to October), Sint Maarten can be hit by tropical depressions and hurricanes, as happens in general in the Caribbean.

Philipsburg

In Sint Maarten, the sea is warm enough for swimming all year round, even though it is a bit warmer in the summer months, as we can see from the following table.
Sea temperature - Sint Maarten
Sint MaartenJanFebMarAprMayJunJulAugSepOctNovDec
Sint MaartenJFMAMJJASOND
Sea (°C)262626262728282829292827
Sea (°F)797979798182828284848281

When to go

The best time to visit the island where Sint Maarten is located goes from December to April, being the least hot and outside the hurricane season, in particular the period from February to March is usually the least rainy. In May, both temperature and moisture increase, but the rains are not yet too abundant, and we are still out of the hurricane season. In summer, June and July are less rainy than August and September, besides tropical storms and hurricanes are less likely to occur than in the following months.

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.