Map from Google - Antigua_and_Barbuda

In Antigua and Barbuda, the climate is tropical, hot all year round. Antigua and Barbuda is a small state belonging to the Commonwealth, and its capital city is St. John's.
The average daily temperature is around 25 °C in January and February, while it goes up to 28 °C from June to October. The breezes temper the heat, which, however, is sultry in the summer months. The heat is more intense especially from July to September.
Here are the average temperatures of St. John's.
Average temperatures - St. John's
St. John'sJanFebMarAprMayJunJulAugSepOctNovDec
St. John'sJFMAMJJASOND
Min (°C)222223232525252625242423
Max (°C)282829293031313131313029

The rainfall amount is around 1,000 millimetres per year, therefore it's not so high. The rainiest period, in which precipitation is equal to or exceeds 100 mm per month, runs from August to November, while the driest period runs from January to March, when precipitation does not exceed 60 mm per month. The rains occur throughout the year especially in the form of downpour or thunderstorm, which soon leaves room for the return of the sun, except when tropical cyclones, called hurricanes in the Caribbean, arrive. These may form from June to November, but are most likely from August to October.
Here is the average precipitation.
Average precipitation - St. John's
St. John'sJanFebMarAprMayJunJulAugSepOctNovDecYear
St. John'sJFMAMJJASONDY
Prec. (mm)55454570906085100130140135851050
Days1197798121312131212125

Antigua

The sea temperature ranges from 26 °C from January to April, to 29 °C in September, so it's warm enough for swimming all year round.
Sea temperature - St. John's
St. John'sJanFebMarAprMayJunJulAugSepOctNovDec
St. John'sJFMAMJJASOND
Sea (°C)262626262728282829282827

When to go

The best time to visit Antigua and Barbuda goes from December to April, being the driest and the least hot; in particular, the period from February to April, which is by far the least rainy, is preferable. In the summer months, August and September are a bit rainier compared with June and July.

What to pack

From December to April: light clothing, possibly a sweatshirt for the evening from December to March.
From May to November: lightweight clothing, possibly a light sweatshirt and a light raincoat for thunderstorms.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.