Map from Google - Grenada

The climate of Grenada is tropical climate, 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.
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. As you can see, there is little variation throughout the year.
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-western coast, almost 2,000 millimetres (78 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 65/70 mm (2.6/2.8 in) per month on average.
Here is the average precipitation in George's.
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

In the southern Grenadines, administered by Grenada (Carriacou, Petite Martinique and some uninhabited islands), the rainfall pattern is similar, though they are a bit drier, in fact they receive less than 1,500 mm (60 in) of rain per year. Even at Point Salines, in the southwestern tip of the main island, where the airport is located, being a flat area and away from inland elevations, precipitation drops below 1,500 mm per year.
The interior of the main island is occupied by hills and low mountains, whose highest point is the volcano St. Catherine, 840 meters (2,760 feet) high, where a rainforest grows, and 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.
The rains in Grenada and the Grenadines occur throughout the year usually 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, at least on the coasts (in which there are about 7/8 hours of sunshine per day all year round), 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, 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; possibly 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, light sweatshirt, raincoat and hiking shoes.