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 (73 °F) in the period from January to March, to 26 °C (79 °F) in the period from June to September, while the maximum goes from 29 °C (84 °F) to 32 °C (90 °F). In the hot and muggy period, however, sea breezes temper the heat.
Here are the average temperatures of the capital Gustavia.
Average temperatures - Gustavia
GustaviaJanFebMarAprMayJunJulAugSepOctNovDec
GustaviaJFMAMJJASOND
Min (°C)232323242526262626262524
Max (°C)292929303132323232313130
Min (°F)737373757779797979797775
Max (°F)848484868890909090888886

Precipitation is not abundant, since it amounts to about 1,000 millimetres (40 inches) per year. In the wettest months, from August to November, from 115 to 130 mm (4.5 to 5 in) of rain per month fall. From January to April, the driest period, the rainfall hovers around 45/65 mm (1.7/2.6 in) per month.
Here is the average precipitation.
Average precipitation - Gustavia
GustaviaJanFebMarAprMayJunJulAugSepOctNovDecYear
GustaviaJFMAMJJASONDY
Prec. (mm)65455565906080115125130125951050
Prec.(in)2.61.82.22.63.52.43.14.54.95.14.93.741.3
Days129891210111214141313137

The amount of sunshine is good in all seasons, because the rains are usually intense but short-lived, so the sun often shines throughout the year.
Sunshine - Gustavia
GustaviaJanFebMarAprMayJunJulAugSepOctNovDec
GustaviaJFMAMJJASOND
Sun (hours)889988898888

From June to November, in Saint-Barthélemy as in all the Caribbean, 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.
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 sea always warm, you can swim all year round.
Sea temperature - St.Barths
St.BarthsJanFebMarAprMayJunJulAugSepOctNovDec
St.BarthsJFMAMJJASOND
Sea (°C)262626262728282829292827
Sea (°F)797979798182828284848281

When to go

The best period to visit Saint-Barthélemy, being the least hot and rainy, runs from December to April. The driest months are February and March. 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 June and July, we are by now in the hot and muggy season, although the rains are less frequent than in August and September, and hurricanes are less likely to occur.

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