Map from Google - Aruba

In Aruba the climate is tropical, hot all year round: daytime temperatures hover around 29/31 degrees Celsius (84/88 °F) throughout the year, the air is muggy, but the wind tempers the heat.
Here are the average temperatures of the capital Oranjestad.
Average temperatures - Oranjestad
OranjestadJanFebMarAprMayJunJulAugSepOctNovDec
OranjestadJFMAMJJASOND
Min (°C)242425252626262626262625
Max (°C)292930303131313131313029
Min (°F)757577777979797979797977
Max (°F)848486868888888888888684

Precipitation is scarce, so that in an average year, about 500 millimetres (20 inches) of rain fall; the only fairly rainy period goes from October to December, when 80/95 millimetres (3/3.7 inches) or rain per month fall, due to the north-east trade winds, which can cause rain showers quite frequently even in January and in the first half of February. The driest months are March, April and May, with less than 10/15 mm (0.3/0.8 inches) per month.
Here is the average precipitation.
Average precipitation - Oranjestad
OranjestadJanFebMarAprMayJunJulAugSepOctNovDecYear
OranjestadJFMAMJJASONDY
Prec. (mm)402010101520302545809580470
Prec.(in)1,60,80,40,40,60,81,211,83,13,73,118,5
Days8522235447111165

Aruba is a flat island, and due to the scarcity of rain water and streams, the water is desalinated from the sea.
The sunshine amount is good all year round, as can be seen in the following table.
Sunshine - Oranjestad
OranjestadJanFebMarAprMayJunJulAugSepOctNovDec
OranjestadJFMAMJJASOND
Sun (hours)8888891099887

The trade winds blow constantly, and the east coast is affected by strong sea currents, while the western and southern coasts are more sheltered, and made of sandy beaches.
From June to November, but especially from August to October, there is the danger of tropical storms and hurricanes, which, however, at these latitudes are rare, because Aruba is located at the southern edge of the hurricane belt. Anyway, it's not impossible that hurricanes could hit Aruba, as happened for example with Hurricane Felix in September 2007.
Aruba is a constituent part of the Netherlands. It's catalogued together with Curaçao, Bonaire and some islands of Venezuela among the Leeward Antilles, namely those islands that in comparison with the rest of the Caribbean have an arid climate, as evidenced by the vegetation consisting mainly of shrubs and cacti. The three islands that once belonged to the Netherlands Antilles (Aruba, Curaçao and Bonaire) are sometimes called ABC Islands.

Aruba

For swimming, the sea is warm enough throughout the year: the water temperature goes from 26 °C (79 °F) from January to April, to 29 °C (84 °F) in October.
Sea temperature - Oranjestad
OranjestadJanFebMarAprMayJunJulAugSepOctNovDec
OranjestadJFMAMJJASOND
Sea (°C)262626262727272828292827
Sea (°F)797979798181818282848281

When to go

The best time to visit Aruba goes from mid-February to May, being the least rainy, and also outside of the period of hurricanes. The period from May to October is more sultry, even though the trade winds make the heat bearable. In the period from October to December, rainfall is moderate but not excessive, and at times this relatively rainy period continues until January or mid-February.

What to pack

All year round: tropics-friendly, light 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.