Map from Google - Bonaire

The climate of Bonaire is tropical, hot all year round, since the temperature fluctuates between 23 °C and 25 °C (73 °F and 77 °F) at night, and between 29 °C and 31 °C (84 °F and 88 °F) during the day. The hottest and more sultry period runs from May to October. However, the north-east trade winds blow constantly, tempering the heat.
Bonaire is a Caribbean island, about a hundred kilometres (60 miles) north of the coast of Venezuela, and it's an official part of the Netherlands, like the neighbouring Curaçao and Aruba.
Here are the average temperatures of the capital Kralendijk.
Average temperatures - Kralendijk
KralendijkJanFebMarAprMayJunJulAugSepOctNovDec
KralendijkJFMAMJJASOND
Min (°C)232323242425252525252423
Max (°C)292929303031313131313029
Min (°F)737373757577777777777573
Max (°F)848484868688888888888684

Being in the Leeward Antilles, the climate is also sunny and dry, so much so that in a typical year, only 565 millimetres (22 inches) of rain fall, with a relatively rainy period between October and December, when, however, rainfall remains below 110 mm (4.3 in) per month. The rains, brought by the northeast trade winds, continue with some frequency also in January or mid-February.
Here is the average precipitation.
Average precipitation - Kralendijk
KralendijkJanFebMarAprMayJunJulAugSepOctNovDecYear
KralendijkJFMAMJJASONDY
Prec. (mm)5025202520254035309510595565
Prec.(in)210.810.811.61.41.23.74.13.722.2
Days9533346557111172

The sunshine amount is good all year round, as can be seen in the following table.
Sunshine - Kralendijk
KralendijkJanFebMarAprMayJunJulAugSepOctNovDec
KralendijkJFMAMJJASOND
Sun (hours)899889999888

Bonaire is a flat island, with a certain shortage of fresh water, because of the lack of rivers.

Bonaire

For swimming, the sea is warm enough throughout the year: the water temperature goes from 26 °C (79 °F) in February and March, to 29 °C (84 °F) in September and October.
Sea temperature - Bonaire
BonaireJanFebMarAprMayJunJulAugSepOctNovDec
BonaireJFMAMJJASOND
Sea (°C)272626272728282829292827
Sea (°F)817979818182828284848281

When to go

The best time to visit Bonaire runs from mid-February to May. The period from May to October is sultry, even though the trade winds make the heat bearable. From June to November, but especially from August to October, there is the danger of tropical storms and hurricanes: compared with the rest of the Caribbean, they are relatively rare at these latitudes, but they can still reach the Leeward Antilles and Bonaire. From October to December the rainfall is moderate, and sometimes the rains, though not excessive, linger on until January or mid-February.

What to pack

All year round: light tropical clothes, made of cotton or linen, 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.