Map from Google - Saint_Lucia

Saint Lucia, an island of the Lesser Antilles belonging to the Commonwealth, with Castries as the capital, has a hot and humid climate throughout the year, with limited temperature variations throughout the year, even though we can divide it into a warmer and sultry period from May to October, and a cooler period from November to April. The minimum temperature goes from 74 °F in January and February, to 77 °F from May to October, while the maximum goes from 84 to 88 °F. All year round, however, the breezes blow, tempering a little the heat.
Here are the average temperatures of Vieux Fort.
Average temperatures - Vieux Fort
Vieux FortJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)737375757777777777777575
Max (°F)848484868888888888888684

The rains are abundant in Saint Lucia: in Vieux Fort, the driest point on the south coast, 55 inches of rain per year fall, of which more than 4 inches per month from July to December. The least rainy months are February and March, with approximately 2/2.5 inches per month on average. Other coastal areas, like the capital Castries, are more rainy, and receive about 80 in of rain, although the rainfall pattern is the same.
Here is the average precipitation in Castries.
Average precipitation - Castries
Prec. (in)4.93.733.

In inland areas, on the slopes of Mount Gimie, rains are more abundant, so that a rainforest grows.
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 hit the island. These may occur from June to November, but are most likely from August to October, as in general in the Caribbean.

Saint Lucia

For swimming, the sea is warm throughout the year: the temperature goes from 79 °F in February and March, to 84 °F in September and October.
Sea temperature - St. Lucia
St. LuciaJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°F)817979818282828284848281

When to go

The best time to visit Saint Lucia goes from February to April, which is the driest of the year; 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; possibly a light sweater for the evening from December to February.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.