Map from Google - Dominica

Dominica, an island of the Lesser Antilles belonging to the Commonwealth, 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 of Roseau, the capital. As you can see, there are little variations between the coolest and the warmest period.
Average temperatures - Roseau
RoseauJanFebMarAprMayJunJulAugSepOctNovDec
RoseauJFMAMJJASOND
Min (°C)222222232425252424232322
Max (°C)282828293030303130293029
Min (°F)727272737577777575737372
Max (°F)828282848686868886848684

The rains are abundant: in Roseau, on the west coast, 2,000 millimetres (78 inches) of rain per year fall, of which more than 200 mm (8 in) per month from July to November. The least rainy period goes from February and April, with about 60/75 mm (2.4/3 in) per month on average. Here is the average precipitation in Roseau.
Average precipitation - Roseau
RoseauJanFebMarAprMayJunJulAugSepOctNovDecYear
RoseauJFMAMJJASONDY
Prec. (mm)125757560952002752602252002251601975
Prec.(in)4.9332.43.77.910.810.28.97.98.96.377.8
Days16913101115222216161816184

The east coast, exposed to the trade winds, receives more rain than the western one, around 2,500 mm (98 in) per year. In the interior of the island, entirely covered by hills and mountains, the rains are even more frequent and abundant, in fact there are rainforests, and the climate can be defined as equatorial. In the northern part of the island we find the volcano Morne Diablotins, 1,447 metres (4,747 feet) high, which receives even 9,000 mm (354 in) of rain per year, while in the south we find the Morne Trois Pitons National Park, a UNESCO World Heritage Site, where, in addition to an impenetrable jungle, we find geysers, rivers and waterfalls, and a hot lake, the Boiling Lake.
The rains in Dominica occur throughout the year as 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. An hurricane that has been particularly destructive was David, which struck the island at the end of August 1979.

Roseau

For swimming, the sea is warm all year round: the water temperature varies between 26 °C (79 °F) in February and March, and 29 °C (84 °F) in September and October.
Sea temperature - Dominica
DominicaJanFebMarAprMayJunJulAugSepOctNovDec
DominicaJFMAMJJASOND
Sea (°C)272626272828282829292827
Sea (°F)817979818282828284848281

When to go

The best time to visit Dominica goes from February to April, being the least rainy. December and January are still quite rainy, though less than the period from July to November, which has the greatest risk of heavy showers and thunderstorms (as well as of hurricanes). It is worth while to recall that the west coast, where Roseau is located, is more sheltered.

What to pack

All year round: light, tropics-friendly clothes of natural fabric, sun hat, sunscreen, possibly a light sweatshirt and a light raincoat for thunderstorms. From December to March, a light sweatshirt for the evening.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes. For hiking in the mountains, hiking shoes, raincoat and sweatshirt.