Map from Google - United_States_Virgin_Islands

The climate of the U.S. Virgin Islands is tropical, hot all year round, with a relatively cooler period from December to April, with average highs around 28/29 °C (82/84 °F), and a hotter and sultry period from May to November, with highs around 31/32 °C (88/90 °F). However, the heat is always tempered by the breeze.
The U.S. Virgin Islands are an archipelago located east of Puerto Rico, in the Lesser Antilles.
Here are the average temperatures of Charlotte Amalie, the capital.
Average temperatures - Charlotte Amalie
Charlotte AmalieJanFebMarAprMayJunJulAugSepOctNovDec
Charlotte AmalieJFMAMJJASOND
Min (°C)222223232525262625252423
Max (°C)293030303132323232323130
Min (°F)727273737777797977777573
Max (°F)848686868890909090908886

As regards the rainfall, there is a rainy season from May to December, with a relative decrease in precipitation in June and July. The least rainy months are February and March. The rainfall hovers around 1,000/1,200 millimetres (40/47 inches) per year, and is generally slightly more abundant in the northern slopes of each island.
In the northernmost islands (Saint John and Saint Thomas, where the capital Charlotte Amalie is located), the relatively dry season runs from December to April, even though during some years there have been heavy rains also in December, from 200 to 400 mm (8 to 16 in), while in the island of Saint Croix, located more to the south, it runs from January to April.
Starting from May, precipitation increases, though often in June and July there is a new relative decrease, followed by a recovery from August to November. Generally, the wettest period runs from September to November.
Here is the average precipitation in Charlotte Amalie.
Average precipitation - Charlotte Amalie
Charlotte AmalieJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)5035357080706595140155140701005

From June to November, the U.S. Virgin Islands, as is the case in general with the Antilles, can be affected by tropical cyclones, called hurricanes in this region. If we exclude these phenomena, which can bring strong winds and torrential rains, normally the weather is nice or variable, interrupted only by brief showers and thunderstorms, which are more likely and more intense during the rainy season: so, the sun shines for many hours throughout the year, as you can see in the following table.
Sunshine - Charlotte Amalie
Charlotte AmalieJanFebMarAprMayJunJulAugSepOctNovDec
Charlotte AmalieJFMAMJJASOND
Sun (hours)889988998877

Charlotte Amalie

For swimming, the sea is warm all year round: the water temperature is around 26 °C (79 °F) from January to April, and 29 °C (84 °F) in September and October.
Sea temperature - Charlotte Amalie
Charlotte AmalieJanFebMarAprMayJunJulAugSepOctNovDec
Charlotte AmalieJFMAMJJASOND
Sea (°C)262626262728282829292827
Sea (°F)797979798182828284848281

When to go

The best time to visit the U.S. Virgin Islands goes from December to April, being the least hot and outside of the hurricane period; in particular January, February and March are the driest months. December can still be rainy, especially in the southern island of Saint Croix. In April, there is a certain increase in temperatures, and during some years also in the intensity of thunderstorms, but overall it's still an acceptable month, and in many cases it is even good. In summer, June and July are generally less rainy than August and September, and present also a lower risk of tropical storms and hurricanes, which are most likely between August and October, when the sea is warmer and has more energy to provide.

What to pack

From December to April: light clothes, possibly a scarf for the breeze and a sweatshirt for the evening.
From May to November: lightweight clothing, possibly a light sweatshirt and a light raincoat for thunderstorms; sweatshirt and pashmina for airconditioned places.