Map from Google - Saint_Lucia

Saint Lucia, an island of the Lesser Antilles belonging to the Commonwealth, with Castries as the capital, has a tropical climate, hot 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 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 December 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).
Here are the average temperatures in Vieux Fort. As you can see, there are little variations between the coolest and the warmest period.
Average temperatures - Vieux Fort
Vieux FortJanFebMarAprMayJunJulAugSepOctNovDec
Vieux FortJFMAMJJASOND
Min (°C)232324242525252525252424
Max (°C)292929303131313131313029
Min (°F)737375757777777777777575
Max (°F)848484868888888888888684

The rains in Saint Lucia vary depending on altitude and location, so that the inland elevations receive more rainfall than the coast, and the areas closest to the mountains more rainy than the ones more distant; however, the rains follow the same pattern, being more frequent and abundant from July to November, and less frequent and shorter from February to April. In Vieux Fort, which is located on the south coast, in the driest point, 1,400 millimetres (55 inches) of rain per year fall, of which more than 100 mm (4 in) per month from July to December. The least rainy months are February and March, with approximately 50/60 mm (2/2.4 in) per month on average. Other coastal areas, such as the capital Castries, receive more rain, about 2,000 mm (79 in) per year, although the pattern is the same.
Here is the average precipitation in Castries.
Average precipitation - Castries
CastriesJanFebMarAprMayJunJulAugSepOctNovDecYear
CastriesJFMAMJJASONDY
Prec. (mm)1259575901252002452052252602151602020
Prec.(in)4.93.733.54.97.99.68.18.910.28.56.379.5
Days14910101115181617201816174

In inland areas, on the slopes of Mount Gimie, 950 metres (3,120 feet) high, the rains are more abundant, and they are covered by rainforests.
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 some more intense waves of bad weather.
However, 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 Lucia

For swimming, the sea is warm throughout the year: the water temperature goes from 26 °C (79 °F) in February and March, to 29 °C (84 °F) in September and October.
Sea temperature - St. Lucia
St. LuciaJanFebMarAprMayJunJulAugSepOctNovDec
St. LuciaJFMAMJJASOND
Sea (°C)272626272828282829292827
Sea (°F)817979818282828284848281

When to go

The best time to visit Saint Lucia goes from December to April, being the coolest and the least rainy, and especially from February to April, which is the dryest of all.

What to pack

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