Map from Google - Saint_Vincent_and_Grenadines

In Saint Vincent and the Grenadines, the climate is tropical, with a high temperature throughout the year, since the maximum goes from 29 °C in January and February, to 31 °C from May to October.
Here are the average temperatures of the capital Kingstown.
Average temperatures - Kingstown
KingstownJanFebMarAprMayJunJulAugSepOctNovDec
KingstownJFMAMJJASOND
Min (°C)242424252624242626262525
Max (°C)292929303130313131313030

Saint Vincent and the Grenadines is an archipelago, made up of no less than 125 islands, most of which are very small. The capital city, Kingstown, is located on the main island, St Vincent. In this island, the volcano La Soufrière is found, 1,234 metres high, whose slopes are often covered by 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 of rain per year fall, including more than 200 mm from June to November. The only months in which the rainfall amount is lower than 100 mm are February, March and April.
Here is the average precipitation.
Average precipitation - Kingstown
KingstownJanFebMarAprMayJunJulAugSepOctNovDecYear
KingstownJFMAMJJASONDY
Prec. (mm)1109090701252052102552252052051751965
Days1410991115182019191816178

The rains occur throughout the year especially in the form of downpour or thunderstorm, and therefore do not reduce too much the sunshine hours, except when hurricanes or tropical storms move over the islands. These intense meteorological phenomena may occur from June to November, but are most likely from August to October, as generally in the Antilles.

Saint Vincent and the Grenadines

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

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.