Map from Google - Antigua_and_Barbuda

Antigua and Barbuda has a tropical climate, hot and humid all year round, with a relatively cool and dry season from January to mid-April, and a hot, humid and rainy season from mid-June to mid-November. In the former period, the northeast trade winds, constant winds typical of tropical climates, blow steadily and with moderate intensity, while in the latter period the winds are more irregular, and may have some breaks, increasing the feeling of sultriness. Between them there are two transitional periods: from mid November to early January (when the northeast wind starts to blow, the temperature decreases a bit and the weather gets better), and from mid-April to Mid-June (when both the temperature and the frequency of downpours gradually increase).
Antigua and Barbuda is a small state belonging to the Commonwealth. In addition to the two main islands, Antigua and Barbuda, we find some smaller islands, such as Green Island, Guiana Island, Long Island and Redonda, plus some islets.
The average daily temperature is around 25 °C (77 °F) in January and February, while it goes up to around 28 °C (82.5 °F) from June to September. Here are the average temperatures of St. John's, the capital, located in the island of Antigua.
Average temperatures - St. John's
St. John'sJanFebMarAprMayJunJulAugSepOctNovDec
St. John'sJFMAMJJASOND
Min (°C)222223232525252625242423
Max (°C)282829293031313131313029
Min (°F)727273737777777977757573
Max (°F)828284848688888888888684

The rainfall is not so abundant, because the islands are pretty flat; the only elevation of some importance is Mount Obama, 402 meters (1,319 feet) high, in the southwest of the island of Antigua.
In Codrington, in the island of Barbuda, rainfall amounts to 1,165 mm (46 in) per year, while in Antigua, it amounts to 1,000 mm (40 in) in the north of the island (see St. John's, the airport), to 1.100 mm (43 in) in the southeastern part, to 1,300 mm (51 in) in the southwest, near Mount Obama.
To simplify, you can divide the year into two periods, one dry from January to June, and the other rainy from July to December. The rainiest period runs from August to November, while the driest months are February and March.
Here is the average precipitation in St. John's.
Average precipitation - St. John's
St. John'sJanFebMarAprMayJunJulAugSepOctNovDecYear
St. John'sJFMAMJJASONDY
Prec. (mm)55454570906085100130140135851050
Prec.(in)2.21.81.82.83.52.43.33.95.15.55.33.341.3
Days1197798121312131212125

The rains occur throughout the year 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.
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.

Antigua

The sea temperature ranges from 26 °C (79 °F) from January to April, to 29 °C (84 °F) in September, so the sea is warm enough for swimming all year round.
Sea temperature - St. John's
St. John'sJanFebMarAprMayJunJulAugSepOctNovDec
St. John'sJFMAMJJASOND
Sea (°C)262626262728282829282827
Sea (°F)797979798182828284828281

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 months of February and March are the least rainy of all. In May, both heat and moisture increase, but the rains are not yet abundant (except in certain years), and we are still out of the hurricane season. In the summer months, August and September are a bit rainier in comparison with June and July, and even hurricanes are more likely.

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. Sun hat, sunscreen.