Zanzibar Beaches:
From Dar es Salaam, a large and lively city [but not Tanzania's capital, that's Dodoma], Zanzibar Island [aka Unguja] is a 90 minute ferry ride or a shorter plane trip off the Tanzanian coast. It is 96kms [60 miles] long and 32kms [20 mls] wide.
The island has 25 beaches, interspersed with fishing villages and all backed by appropriately tropical palms or mangroves and fronted by the warm, azure waters of the Indian Ocean. The beaches are mostly huge white strands of sand occasionally garbed in seaweed [in fact seaweed is cultivated here]; Zanzibar's best beaches are on the north and east coasts, best snorkelling too. But note that experienced travellers prefer Mafia Island's more isolated ambience.
Here are a few options:

Bububu beach, on the west coast.
On the southern tip of the island, Kizimkazi fishing village is close to several schools of bottle-nosed dolphins which can often be seen and perhaps swum with after a short boat trip from the village.

A typical inexpensive Zanzibar beach hotel on Matemwe, the east coast.

Prison Island. Built as a slave jail though never used, the island is close to Stone Town. With a fine white beach, some giant tortoises and an excellent coral reef Prison Island is perfect for snoring and snorkelling.

Ras Nungwi, a relaxed fishing village on the north tip of Zanzibar is traditionally a dhow building spot that has morphed recently into Zanzibar's favourite beach resort with swarms of tourists and humming concrete mixers putting up the latest low-rise resort, but still pleasant and with a full complement of warm, turquoise seas and broad bands of white sand, even if they are coated in seaweed in winter time [November-January].
Scuba, snorkelling, wind-surfing and sunset dhow cruises are all available around Nungwi, as is a lively nightlife much favoured by backpackers.
Kendwa beach, south of Nungwi is quieter and - at the moment - way less developed but nevertheless hosts a couple of solid, late night music scenes and a regular beach party/full moon party.
Pemba Island: north of Zanzibar. Just 50kms [31mls] separate the two islands, but few travellers get to Zanzibar's smaller brother Pemba in spite of the relaxed but lively ambience, the ruins stretching back to 1200 AD, the pleasant old town Baixa that wafts with the scents of cloves and voodoo, some terrific beaches - particularly Wimbi [aka Wimbe], and Pemba's raison d'etre, world-class reef or drift diving.
Mafia Island: south of Zanzibar. Unspoilt and also offering great diving, with crystal waters and dazzling white sandbanks; some think that Mafia's Chole Bay is perfect as a year-round dive spot as it is sheltered - with clearer visibility - whereas Pemba is not.
Zanzibar Travel Guide | Tanzania Travel Guide
Best
season for Zanzibar beaches:
Zanzibar is near the equator which makes for a warm and generally month-by-month similar climate.
Best season: June-September.
OK: October-February.
Temperatures normally range from low 20'sC [70's F] to mid 30'sC [100 F], which
is fine, though the humidity can get uncomfortable.
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