Map from Google - Montserrat

Montserrat is an island, overseas territory of the United Kingdom, which has a 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 mid-June to mid-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 mid November 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 the old capital Plymouth; as you can see, there are little variations between the coolest and the warmest period.
Average temperatures - Plymouth
PlymouthJanFebMarAprMayJunJulAugSepOctNovDec
PlymouthJFMAMJJASOND
Min (°C)212121222324242423232322
Max (°C)282829303131313132312928
Min (°F)707070727375757573737372
Max (°F)828284868888888890888482

The rainfall in Montserrat is abundant, amounting to 1,600 millimetres (65 inches) per year in the former capital Plymouth, of which more than 100 mm (4 in) per month from May to January, and more than 150 mm (6 in) from July to November. The driest month is February, with an average of 85 mm (3.3 in), which means that some showers are possible even in this period. The rains continue for a while after the end of the hot and wet period, because in the first months of the north-east trade winds, the sea is still warm enough to provide energy and moisture for the formation of thunderclouds.
The rains in Montserrat 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 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.
Here is the average precipitation in Plymouth.
Average precipitation - Plymouth
PlymouthJanFebMarAprMayJunJulAugSepOctNovDecYear
PlymouthJFMAMJJASONDY
Prec. (mm)1208511090951101551851701951801401640
Prec.(in)4.73.34.33.53.74.36.17.36.77.77.15.564.6
Days129981013141613141613147

The interior of the island is occupied by hills and low mountains, and is even more rainy. The highest point is the volcano Soufrière Hills, 915 metres (3,000 feet) high, which in 1995 produced a strong eruption and devastated the southern part of the island, including the capital Plymouth. The southern part has been declared "exclusion zone", while the northern part is still inhabited, and is believed to be protected from volcanic eruptions.

Montserrat

The sea is pleasantly warm all year round, as we can see from the following table.
Sea temperature - Montserrat
MontserratJanFebMarAprMayJunJulAugSepOctNovDec
MontserratJFMAMJJASOND
Sea (°C)272626272828282829292827
Sea (°F)817979818282828284848281

When to go

The best time to visit Montserrat goes from February to April, being the driest. In December and January, there are some additional showers, but we are still in the least hot period, so all in all they are acceptable months as well.

What to pack

All year round: light clothes of natural fabric, sun hat, sunscreen, 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.