At the moment (2011), Journeys by Design designs Tanzanian coast based itineraries that incorporate the Zanzibar archipelago, Mafia Island and - on the mainland - Dar-es-Salaam and Ras Kutani.
Climatically, the mainland coast, the Zanzibar archipelago and Mafia Island are all influenced by the Kaskazi and Kuzi monsoons. The first of these, the Kaskazi blows in from the north-east during the months of November to March inclusive. Reasonably benign, these relatively dry winds, though warm, help mitigate the effects of an otherwise very humid atmosphere. The second monsoon, the Kuzi, lasts from April through to September, and brings with it the long rains, which last from April through to June, with April being the year’s wettest month. The short rains, though present, are less significant, and occur between October and November.
It is hot wherever you are, mainland or island, with the maximum average temperature being 29.3°C, and the months between October and April reaching into the early thirties. These temperatures are further fired by the levels of humidity, which peak in April (87%) and November (76%), adding muggy and very hot to the list of adjectives with which one could go on to describe the coast based rainy seasons. The long dry season, June through to September, is less humid (60%), the nights more temperate.
While the temperature is constant throughout the area, precipitation levels differ, with the mainland receiving an average of 1,100mm per annum, and Zanzibar between 1,500mm (Unguja) and 1,820mm (Pemba), while Mafia gets as much as 2000mm. Most of the rain falls between March and May. Its quality is heavy, and it can rain every day for two months, which is why many lodges tend to close during this period. The shorter rains are much more manageable, and occur in the afternoon and night, leaving the rest of the day clear and bright.
The best time to visit the coast would be any time between June and the end of February, which is also when the sea is at its calmest. With the Kuzi giving way to the Kaskazi, visibility is excellent between September and March, making this the best time to snorkel and dive. Fishing is similarly good between September to early December and February through to April, with the tuna season being August - October and billfish marlin mid-November through to March.