Map from Google - Haiti

In Haiti the climate is tropical, hot throughout the year, with highs around 29/30 degrees Celsius in the cooler months (November to March) and 32/33 °C in the warmest months (May to October). The temperatures are a bit higher in inland plains and south facing coasts, and slightly lower along the north facing coasts.
From November to March, the trade winds from the northeast prevail.
The warmest season is also more humid, so in this period the heat gets annoying, though along the coast it's tempered by the breezes.
From December to March, however, sometimes the north-west cool wind, coming from the United States, can lower the temperature by a few degrees, bringing the night temperatures below 20 °C.
Here are the average temperatures of Port-au-Prince, located in a particularly sheltered and warm area.
Average temperatures - Port-au-Prince
Port-au-PrinceJanFebMarAprMayJunJulAugSepOctNovDec
Port-au-PrinceJFMAMJJASOND
Min (°C)202021222223232323222221
Max (°C)313132323233343433323131

With regard to 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 capital, Port-Au-Prince, where a total of 1,250 millimetres of rain per year fall.
Here is the average precipitation in Port-au-Prince.
Average precipitation - Port-au-Prince
Port-au-PrinceJanFebMarAprMayJunJulAugSepOctNovDecYear
Port-au-PrinceJFMAMJJASONDY
Prec. (mm)3035751551901106513516516080451245
Days3571113871112127399

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 the average rainfall in Cap-Haïtien.
Average precipitation - Cap Haitien
Cap HaitienJanFebMarAprMayJunJulAugSepOctNovDecYear
Cap HaitienJFMAMJJASONDY
Prec. (mm)13011090120190155951001501702101801700
Days108991212141415161211142

Moving inland, we find some hills and in the south some mountains, among which the highest is Pic la Selle, 2,680 metres high; in this area, there are also sheltered valleys and plains, which are almost dry.
As generally happens in the tropical climate, the rains in Haiti occur mostly in the form of short, intense showers or thunderstorms 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 although rarely hit directly a particular location, they are dangerous and may sometimes be destructive. However, they 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 from February to April, to 29 °C 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
Port-Au-PrinceJanFebMarAprMayJunJulAugSepOctNovDec
Port-Au-PrinceJFMAMJJASOND
Sea (°C)272727272828292929292928

When to go

The best time to visit the capital and the south-central area of Haiti is from December to March. During 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 less worse) is July because it generally sees a certain decrease in rainfall.
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.