Map from Google - Barbados

Barbados, the easternmost of the West Indies, has a tropical climate, hot and humid all year round, a bit cooler and more pleasant from December to April, and more hot and muggy from May to October. However, the heat is tempered by the breezes.
The temperature does not vary much throughout the year, as you can see from the averages of the capital Bridgetown.
Average temperatures - Bridgetown
BridgetownJanFebMarAprMayJunJulAugSepOctNovDec
BridgetownJFMAMJJASOND
Min (°C)212121222324232323232322
Max (°C)282930303131303130303029
Min (°F)707070727375737373737372
Max (°F)828486868888868886868684

Along the coast the annual rainfall amounts to 1,200/1,400 millimetres (47/55 inches), while it's slightly more abundant in inland areas (though not by much, because there are no high elevations that could force air masses to rise). In Bridgetown, more than 100 millimetres (4 inches) of rain per month fall from June to November; the wettest period runs from September to November, with around 170/180 mm (6.7/7.1 in) of rain per month, while the driest runs from February to April, with 35/50 mm (1.4/2 in).
Here is the average precipitation.
Average precipitation - Bridgetown
BridgetownJanFebMarAprMayJunJulAugSepOctNovDecYear
BridgetownJFMAMJJASONDY
Prec. (mm)6540355070105140145170175180951270
Prec.(in)2.61.61.422.84.15.55.76.76.97.13.750
Days12977714191515151413147

The rains, however, are quite irregular from month to month, which means that even in the rainy season, the weather may be good for entire weeks. The rains in Barbados occur especially in the form of downpours or thunderstorms, maybe intense, but which generally do not last long, and therefore do not reduce too much the sunshine hours, though sometimes there may be a more intense wave of bad weather.
In fact, the amount of sunshine is good all year round, as can be seen in the following table.
Sunshine - Bridgetown
BridgetownJanFebMarAprMayJunJulAugSepOctNovDec
BridgetownJFMAMJJASOND
Sun (hours)899988888888

The greatest danger comes from and tropical storms and cyclones, called hurricanes in the Caribbean area, which may pass from June to November, but are most likely from August to October.

Barbados

For swimming, the sea is warm enough all year round, as we can see from the following table.
Sea temperature - Bridgetown
BridgetownJanFebMarAprMayJunJulAugSepOctNovDec
BridgetownJFMAMJJASOND
Sea (°C)272626272828282829282828
Sea (°F)817979818282828284828282

When to go

The best time to travel to Barbados goes from February to April, which is the least rainy of the year, but after all also December and January are acceptable, with a few more showers. In May, heat and moisture increase, but the rains are not yet too abundant, and we are still out of the hurricane season.

What to pack

All year round: light clothes made of cotton or linen, sun hat, sunscreen, possibly a light sweatshirt and a light raincoat for thunderstorms; from December to March, a light sweatshirt for the evening.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.