Map from Google - Curaçao

In Curaçao the climate is tropical, hot all year round, with daytime temperatures ranging from around 82/84 degrees Fahrenheit in December, January and February, to around 88 °F from May to October. The period from May to October is the most sultry, but the trade winds blow constantly, tempering the heat.
Curaçao is an island, like the neighboring Bonaire and Aruba it's part of the Kingdom of the Netherlands, and its capital is Willemstad.
Here are the average temperatures of Willemstad.
Average temperatures - Willemstad
WillemstadJanFebMarAprMayJunJulAugSepOctNovDec
WillemstadJFMAMJJASOND
Min (°F)757577777979797979797977
Max (°F)828486868888888888888684

The climate is sunny and dry, so that about 22 inches of rain per year fall. The only relatively rainy period goes from October to December, in which, however, precipitation does not go beyond 3/4 inches per month. From November to January or mid-February, the northeast trade winds bring showers quite frequently, although usually not abundant.
Here is the average precipitation.
Average precipitation - Willemstad
WillemstadJanFebMarAprMayJunJulAugSepOctNovDecYear
WillemstadJFMAMJJASONDY
Prec. (in)1.810.60.80.80.71.61.71.93.33.83.921.9
Days9533236557101270

Curaçao is an almost entirely flat island: its highest point is located at 1,230 feet above sea level.
From June to November, but especially from August to October, there is the danger of tropical storms and hurricanes, which, however, are rare in Curaçao and the Leeward Antilles, because of their southern location. A hurricane that hit Curaçao along with Aruba and Bonaire was Joan, which passed in October 1988, bringing strong winds and abundant rainfall, and causing widespread flooding.

Curaçao

For swimming and sun bathing, the sea is warm enough throughout the year: the water temperature goes from 79 °F in February and March, to 84 °F in September and October.
Sea temperature - Curaçao
CuraçaoJanFebMarAprMayJunJulAugSepOctNovDec
CuraçaoJFMAMJJASOND
Sea (°F)817979818182828284848281

When to go

The best time to visit Curaçao runs from mid-February to May. The period from October to December (and sometimes even January and the first half of February) is quite rainy.

What to pack

All year round: light, loose fitting clothes, possibly a light raincoat for thunderstorms, especially from October to January; those who are sensitive to cold can bring a light sweatshirt for the evening.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.