Map from Google - Haiti

In Haiti the climate is tropical, hot throughout the year, with a less hot period from November to March, in which the trade winds from the northeast prevail, and a muggy period from May to October, in which the heat becomes unpleasant, though along the coasts it is tempered by the breeze. Temperatures are a bit higher in inland plains and south-facing coasts, and slightly lower along the north-facing coasts.
From December to March, sometimes the north-west cool wind, coming from the United States, can lower the temperature by a few degrees, bringing the night temperatures a few degrees below 20 °C (68 °F).
Here are the average temperatures of the capital, Port-Au-Prince, located in a particularly sheltered and warm area.
Average temperatures - Port-au-Prince
Min (°C)202021222223232323222221
Max (°C)313132323233343433323131
Min (°F)686870727273737373727270
Max (°F)888890909091939391908888

As regards the rains, there is usually a dry season from December to February, and a rainy season from April to October, with with two peaks at the beginning and the end of the period, and a relative pause in July. This is the trend that is observed in the Port-au-Prince, where a total of 1,250 millimetres (49 inches) of rain per year fall.
Here is the average precipitation in Port-au-Prince.
Average precipitation - Port-au-Prince
Prec. (mm)3035751551901106513516516080451245

In other areas, there are microclimates with some variations from this pattern: the coasts exposed to the northeast trade winds are rainy even in winter, especially in the first part, so that the rainiest period goes from November to January. This happens in the northern coast (see Cap-Haïtien), and and to a lesser extent in the north-facing coast of the Department of Grand Anse (see Jérémie).
Here is the average rainfall in Cap-Haïtien.
Average precipitation - Cap Haitien
Cap HaitienJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)13011090120190155951001501702101801700

Moving inland, we find some hills, and in the south also some mountains, among which the highest is Pic la Selle, 2,680 metres (8,793 feet) high; in this area, there are also sheltered valleys and plains, which are almost arid, where precipitation drops below 1,000 mm (40 inches) per year.
As generally happens in tropical climates, the rains in Haiti occur mostly in the form of short but intense showers or thunderstorms, especially in the evening, which do not affect much the amount of sunshine, except perhaps in September and October, which are typically the rainiest months, and when thunderstorms are more frequent and may last longer.
From June to November, in Haiti, as commonly happens the Caribbean, it's the period of hurricanes, which, though rarely directly hit a particular location, are dangerous and may sometimes be destructive. However, hurricanes are most likely from August to October.
The sea in Haiti is warm all year round: on the northern coast, the sea surface temperature goes from 26 °C (79 °F) from February to April, to 29 °C (84 °F) from July to October, while at Port-au-Prince it's just slightly warmer, as we can see from the following table.
Sea temperature - Port-Au-Prince
Sea (°C)272727272828292929292928
Sea (°F)818181818282848484848482

When to go

The best time to visit the capital and the south-central area of Haiti is from December to March. In summer, when there's no shortage of sunshine, but the heat is sweltering, and besides there is the risk of hurricanes, the best month (or the least bad) is July, because it generally sees a certain decrease in rainfall, furthermore, hurricanes are less likely than in August and September.
On the northern coast of Haiti, given the frequent rains lingering on until December, you can choose January, February and March. Even the northern coasts experience a decrease in rainfall in July.
Winter is warm enough for a beach holiday, even though Haiti is not a peaceful country in socio-economic terms, and it's not equipped for this type of activity.

What to pack

From November to April: light clothes, a sweatshirt for the evening; for the mountains, a jacket and hiking boots.
From May to October, lightweight clothing, possibly a light sweatshirt and a light raincoat for thunderstorms; a sweatshirt or sweater for the mountains.