Map from Google - Saint_Vincent_and_Grenadines

In Saint Vincent and the Grenadines, the climate is tropical, 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 June to December. 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 mid-April and late May, before the warmer and rainy season, both the temperature and the frequency of downpours gradually increase.
Here are the average temperatures of the capital Kingstown. As you can see, there are few variations throughout the year.
Average temperatures - Kingstown
KingstownJanFebMarAprMayJunJulAugSepOctNovDec
KingstownJFMAMJJASOND
Min (°C)242424252624242626262525
Max (°C)292929303130313131313030
Min (°F)757575777975757979797777
Max (°F)848484868886888888888686

Saint Vincent and the Grenadines is an archipelago, made up of no less than 125 islands, most of which are very small. The main islands of the Grenadines are Bequia, Mustique, Canouan, Mayreau, Union Island, Palm Island, Petit Saint Vincent. The capital city, Kingstown, is located on the main island, St Vincent. In this island, we find the volcano La Soufrière, 1,234 metres (4,049 feet) high, whose slopes are often covered with clouds. On the contrary, on the coast the climate is more sunny.
The rainfall is abundant, especially on the main island. In Kingstown, almost 2,000 millimetres (78 inches) of rain per year fall, including more than 200 mm (8 in) from June to November. The only months in which the rainfall amount is lower than 100 mm (4 in) are February, March and April.
Here is the average precipitation.
Average precipitation - Kingstown
KingstownJanFebMarAprMayJunJulAugSepOctNovDecYear
KingstownJFMAMJJASONDY
Prec. (mm)1109090701252052102552252052051751965
Prec.(in)4.33.53.52.84.98.18.3108.98.18.16.977.4
Days1410991115182019191816178

On the contrary, the Grenadines are a bit drier, in fact they receive less than 1,500 mm (60 in) of rain per year.
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.

Saint Vincent and the Grenadines

The sea is warm enough for bathing throughout the year: the temperature goes from 27 °C (81 °F) from January and April, to 29 °C (84 °F) in August and September.
Sea temperature - Kingstown
KingstownJanFebMarAprMayJunJulAugSepOctNovDec
KingstownJFMAMJJASOND
Sea (°C)272727272828282929292828
Sea (°F)818181818282828484848282

When to go

The best time to visit Saint Vincent and the Grenadines goes from February to April, since it's the least rainy of the year, and outside the period of hurricanes; all in all even January is an acceptable month, although some additional showers must be put into account.

What to pack

All year round: light clothes of natural fabric, possibly a light sweatshirt and a light raincoat for thunderstorms; a light sweatshirt for the evening from December to March.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
For hiking in mountain ranges, sweatshirt, raincoat and hiking shoes.