Map from Google - Suriname

Suriname, the former Dutch Guiana, has almost everywhere an equatorial climate, hot and humid throughout the year, with heavy rains, in fact it is covered with rainforests and mangroves.
However, in most of the country there is a rainy season from December to mid-August (with a relative decrease in February and March and a peak in May and June), and a relatively dry season from late August to mid-November.
In the capital Paramaribo, located on the coast, an average of 2,200 mm of rain falls per year; the rainiest months are May and June, when the rainfall reaches 300 mm per month, and when it rains almost every day. Here is the average precipitation.
Average precipitation - Paramaribo
ParamariboJanFebMarAprMayJunJulAugSepOctNovDecYear
ParamariboJFMAMJJASONDY
Prec. (mm)19515016023032030522516585851101752205
Days1813141623232014991218189

The rainiest regions are the east, along the Maroni River on the border with French Guyana, where the average precipitation reaches 2,500 millimetres per year, and the south-central, at the foot of the Wilhelmina mountains, whose highest point is Julianatop, 1,280 metre) high, where the rainfall reaches 3,000 mm per year.
In the far south, in the Sipaliwini Savanna Nature Reserve, the dry season is longer, so the climate becomes tropical, and the land is covered by savannah, as the name suggests. Here the rainfall stays below 100 mm per month from September to December, and just above this level in August, January and February.
As mentioned, in Suriname it's hot all year round. In Paramaribo, the average daily temperature ranges from 26 degrees Celsius in the cooler months (December to March), to 28 °C in the hottest months (September and October).
Here are the average temperatures of Paramaribo.
Average temperatures - Paramaribo
ParamariboJanFebMarAprMayJunJulAugSepOctNovDec
ParamariboJFMAMJJASOND
Min (°C)222222222322222323232322
Max (°C)303030313031313233333230

The sea in Suriname is warm all year round, and its temperature is similar to the daily average on the coast, oscillating between 26 °C and 28 °C.
Sea temperature - Paramaribo
ParamariboJanFebMarAprMayJunJulAugSepOctNovDec
ParamariboJFMAMJJASOND
Sea (°C)262727272728282828282827

The amount of sunshine is not so high from December to May, when there are on average 6 hours of sunshine per day in Paramaribo, while it is good in the driest period, late August to mid-November, when there are 9 hours of sunshine per day.
Suriname lies outside of the hurricane belt, because they pass over the Caribbean, hence north of the country.

Paramaribo

When to go

The best time to visit Suriname runs from late August to mid-November, which is the driest and sunniest; even though it is also the hottest, because here it's more advisable to avoid the heavy rains.
The little dry season, from February to March, is less pronounced, but it is still quite noticeable at least along the north coast, especially from Paramaribo to the west, that is, to the border with Guyana. This is however a less sunny period than the former, and besides, depending on the year, the rains can sometimes be heavy.
In the far south, the Sipaliwini Savanna can be visited for a longer period, from August to February, with a preference for the period from September to December.

What to pack

Throughout the year: tropics-friendly, loose fitting clothing, light shirts and pants of natural fibres (cotton, linen) or synthetics that breathe, to be washed often, maybe with long sleeves for mosquitoes; a light sweatshirt for the evening; sun hat; umbrella or light and breathable raincoat.
From December to March, a sweater for the evening.