Map from Google - Saint_Barthelemy

In Saint-Barthélemy, the climate is tropical, hot all year round, with a relatively cool season, from December to April, called "the Lent", and a warm and humid season, from May to November, called "the Wintering", though the latter actually resembles everything but winter, because it is a sultry period, in which the sun shines, but there are also showers or thunderstorms, especially in the afternoon.
The temperature varies little throughout the year: the minimum goes from 23 degrees Celsius in the period from January to March, to 26 °C in the period from June to September, while the maximum goes from 29 °C to 32 °C. In the hot and muggy period, however, sea breezes temper the heat.
Here are the average temperatures of the main city Gustavia.
Average temperatures - Gustavia
GustaviaJanFebMarAprMayJunJulAugSepOctNovDec
GustaviaJFMAMJJASOND
Min (°C)232323242526262626262524
Max (°C)292929303132323232313130

Precipitation is not abundant, since it amounts to about 1,000 millimetres per year. In the wettest months, from August to November, from 115 to 130 mm of rain per month fall. From January to April, the driest period, from 45 to 65 mm of rain per month fall.
Here is the average precipitation.
Average precipitation - Gustavia
GustaviaJanFebMarAprMayJunJulAugSepOctNovDecYear
GustaviaJFMAMJJASONDY
Prec. (mm)65455565906080115125130125951050
Days129891210111214141313137

The amount of sunshine is good throughout the year, because the rains are usually intense but short-lived, so the sun often shines all over the year.
From June to November, in the Caribbean and therefore also in Saint-Barthélemy, there is the danger of tropical storms and hurricanes, which however are more likely from August to October.
Saint-Barthélemy (also known as Saint-Barth or St. Barts) is a small island, which before 2007 was part of the French overseas department of Guadeloupe, while before 2003 it was an overseas territory of France. Its capital is Gustavia.
Because of the lack of mountains and rivers, there is a certain shortage of water available, therefore the water is collected from the rain, desalinated from the sea or obtained from incinerators.

Saint-Barthélemy

In Saint-Barthélemy, as generally happens in the Lesser Antilles, being the seawater always warm, you can swim all year round.
Sea temperature - St.Barths
St.BarthsJanFebMarAprMayJunJulAugSepOctNovDec
St.BarthsJFMAMJJASOND
Sea (°C)262626262728282829292827

When to go

The best period to visit Saint-Barthélemy, being the least hot and rainy, runs from January to April. The absolute least rainy months are February and March. Also in June it usually rains little, but we are now in the hot and muggy season, and also at the beginning of the hurricane season.

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 December; a light sweater or sweatshirt for the evening from December to March.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.