Map from Google - Grenada

The climate of Grenada is hot and humid all year round, with limited variations in temperature: lows are around 22 °C (72 °F) from December to March, and 24 °C (75 °F) from May to October, while highs go from 30 °C to 31 °C (86 to 88 °F). The period from May to October is also the most humid and uncomfortable.
Grenada is a country of the Lesser Antilles, located halfway between the Equator and the Tropic of Cancer, and includes the main island plus the smaller southern Grenadines.
Here are the average temperatures of the capital St. George's.
Average temperatures - St. George's
St. George'sJanFebMarAprMayJunJulAugSepOctNovDec
St. George'sJFMAMJJASOND
Min (°C)222222232424242424242323
Max (°C)303031313131313131313130
Min (°F)727272737575757575757373
Max (°F)868688888888888888888886

The rains are abundant: in St. George's, on the south coast, almost 2,000 millimetres (79 inches) of rain per year fall, of which more than 100 mm (4 in) per month from May to January, and more than 200 mm (8 in) from June to November. The least rainy months are March and April, with 60/70 mm (2.4/2.8 in) per month on average. The rainfall pattern is similar in the southern Grenadines, which are a bit drier, with about 1,500 millimetre) of rain per year. In inland areas of the main island, on the mountain slopes, where a rainforest grows, we can speak of equatorial climate: precipitation is abundant throughout the year, reaching even 3,500 mm (138 in) per year, the sky is often cloudy, and the temperature is a bit lower.
Here is the average precipitation.
Average precipitation - St. George's
St. George'sJanFebMarAprMayJunJulAugSepOctNovDecYear
St. George'sJFMAMJJASONDY
Prec. (mm)1158065701152202302202002452451751980
Prec.(in)4.53.12.62.84.58.79.18.77.99.69.66.978
Days1491071317201817202018183

The rains in Grenada and the Grenadines occur throughout the year usually in the form of downpour or thunderstorm, and therefore do not reduce too much the sunshine hours, at least on the coasts (in which there are about 7/8 hours of sunshine per day all year round), except when the islands are hit by tropical storms and hurricanes. These can form from June to November, but are most likely from August to October, as in all the Caribbean, although Grenada is located in the southernmost part of the hurricane belt, so it is rarely affected by them.

Grenada

For swimming, the sea is warm throughout the year: the temperature goes from 27 °C (81 °F) in the period from December to April, to 29 °C (84 °F) in September and October, as we can see from the following table.
Sea temperature - Grenada
GrenadaJanFebMarAprMayJunJulAugSepOctNovDec
GrenadaJFMAMJJASOND
Sea (°C)272727272828282829292827
Sea (°F)818181818282828284848281

When to go

The best time to visit Grenada goes from February to April, which is the driest of the year, but all in all even January is an acceptable month.

What to pack

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