Map from Google - Zanzibar

Zanzibar is an archipelago that is part of Tanzania, from which was once independent and from which still maintains a certain autonomy; the main islands are Unguja (also called Zanzibar) and Pemba.
The climate is tropical, hot all year round, with two rainy seasons: one more intense, known as "long rains season" from March to May, with the peak in April, and the other less intense, known as "short rains season", between mid-October and December. The total annual rainfall is about 1,600 millimetres (63 inches) in Zanzibar, and 1,900 mm (75 in) in Pemba. April and May are the wettest months, in which there can be can be really strong downpours. However, a few short thunderstorms in the afternoon can occur throughout the year.
Here is the average precipitation in Zanzibar.
Average precipitation - Zanzibar
Prec. (mm)855515038525570505050952151851645

The warmest period is from October to March, during which the maximum temperatures hover around 31/33 degrees Celsius (88/91 °F) and the humidity is high, although the breeze temper a little the heat. In the worst moments, the temperature can reach 36/38 °C (97/100 °F). The period May-August, in which the southeast trade winds predominate, is cooler, with highs around 29 °C (84 °F).
Here are the average temperatures in Zanzibar.
Average temperatures - Zanzibar
Min (°C)232323242322212020212223
Max (°C)323332302929292930313131
Min (°F)737373757372706868707273
Max (°F)909190868484848486888888

Beach in Zanzibar

The sea is always warm enough for swimming: the water temperature ranges from 25 °C (77 °F) in August, to 29 °C (84 °F) from January to April.
Sea temperature - Zanzibar
Sea (°C)292929292827262526262829
Sea (°F)848484848281797779798284

When to go

The best months to enjoy the sun and the sea of Zanzibar are July, August and September. In Zanzibar the rains cease or greatly decrease already in June, so in this island you can also go in June, while in the more northerly island of Pemba, it still rains a bit in this month. If you want, you can also choose the period from January to February, between the two rainy seasons, but this period is hot and sultry.

What to pack

In winter (June to August): light clothes, a sweatshirt for the evening, a light scarf for the breeze, sunscreen. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In summer (December to February): tropics-friendly, loose fitting clothing, a light sweatshirt for the evening, a light raincoat or umbrella until December, sunscreen. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.