About 100km (62 miles) from south to north coast
As beautiful as Zanzibar's coastline is, it is no longer a well-kept secret. The northern and eastern shores, lapping the longest stretch of soft sands, are lined with resorts, many of them sharing the same wall, a jigsaw pattern of narrow rectangles that runs for kilometers at a time when seen from the air. Of course, this is not necessarily how you will experience it. Picked up from the airport and whisked directly to your resort, you may remain blissfully unaware of the proximity of an endless line of neighbors, particularly if you are in an intimate little lodge or (possible albeit rare) take a wander along the beach for half an hour before seeing the next resort.
If a sense of remoteness is important to you, choose one of the two lodges I have included on the south coast. Aside from the rocky western shoreline, the south coast is the least populated part of Zanzibar. Based here, you have the added benefit of being close to Kizimkazi and stand the best chance of swimming with the bottle-nose dolphins that frequent its coastline. Board the boat at 6am and you are unlikely to share the experience with day-trippers who are driven down from Stone Town or from the east coast resorts, who arrive from around 8am onward. The south coast also offers some lovely snorkeling opportunities, predominantly in the Minai Bay conservation area (though it is possible to access this with Safari Blue, regardless of where you are based on the island), and the ocean does not recede for miles at low tide as it does on the east coast. The biggest drawback is the lack of beaches; interspersed among the rocky coastline are small sandy coves, but you don't have the long stretches of the east or north coast. Dive and snorkel sites here are also not the best.
Most people head for popular Ras Nungwi on the northwestern tip. Outside of Stone Town, this is the most densely developed tourist area on the island and, judging by the huge number of resorts built every year, will remain that way well into the next decade. A large sprawling fishing village, more developed than most of the coastal settlements, Ras Nungwi is the dhow-building capital of Zanzibar, with a substantial local population that gives it an edge you won't find in any of the other coastal destinations. While the waters are as azure as on the east coast, this is a place of industry, and the northwestern beach around Z Hotel has small stalls flogging everything from beaded bangles to snorkeling trips, and beach bars vie for customers with music and the lowest seafood and beer prices on the island. As a result, it's by far the most popular destination for budget travelers, with plenty of lodges offering clean but rudimentary accommodation. The whole atmosphere is one of hustle and bustle, and there's a general sense of low-level party. Beaches are less pristine than on the less populated east coast, but the ocean does not recede as far during low tide, and the diving and snorkeling in the area is excellent (don't miss a chance to dive Levan Bank; you are also relatively near Mnemba Atoll off the northeast coast). It's a rare treat to spot a turtle while diving or snorkeling. For a guaranteed encounter, head east from the beach toward the lighthouse, where you'll find the Mnarani Turtle Sanctuary, a fenced-off saline pool in which injured greenback and hawksbill turtles and other marine animals are nursed back to health before being released back into ocean. A little south of Nungwi on the northwestern coast is Kendwa, a smaller fishing settlement experiencing a similar mushrooming of resorts. Near here (look for the signpost along the main road, around 10km/6 1/4 miles south of the village) are the 16th-century Fukuchani Ruins, also known as Portuguese House; it's said to be well preserved and might be a worthwhile stop for history buffs, but for most of us, the real attraction is back on the beach and in the water.
With the longest unbroken beaches on the island, the east coast is understandably Unguja's most exclusive coastline, and with development a great deal more spread out, the tourist pressure is largely dissipated. When the tide is low, however, the ocean recedes a long way back, and a pool is then essential to cool off. There are two distinct areas here: The southeast stretches from Jambiani and Paje, which used to be where the budget travelers congregated, stretches northward to the more remote Ras Michamvi. From here, you need to drive back via Jozani to get to the road that connects Stone Town with the central-eastern coastal road that starts at Chakwa and culminates in the northeast at Matemwa; then you are ideally situated for the island's best snorkeling and diving on the Mnemba Atoll. This eastern coastline is mostly big-resort territory, lined cheek-by-jowl, but, thankfully, there are still a handful of intimate low-key resorts that I recommend.