The Indian Ocean is the perfect place for watching dolphins, and if you spend 4 or 5 days on a sailing yacht, you will have the possibility of seeing several groups of them, and several different species.
The most common kind is the spinner dolphin (Stenella longirostris), especially along the coast of Pemba, where you can often see it in groups of several hundreds, and with young ones. This dolphin likes to play, and will often change direction, when hearing a boat, to swim along. The name spinner comes from the fact, that they jump out of the water, and spin in the air, before falling back in the water, which is very impressive, and always gives us the impression, that they are making a show for us.
The other common species is the Indo-Pacific bottlenose dolphin (Tursiop aduncus), well know of people, as it is the one seen in marinariums. It often travels in smaller groups as the spinner dolphin, but plays at the bow for much longer, and as they are less shy, it is with them, that you will have a possibility to swim, or even snorkel!
There is a third species, which we get to see from time to time along the coastal area, at dawn or sunset, and it is the very shy humpback dolphin (Sousa chinensis). This dolphin lives in very small groups, in shallow water, quiet areas, close to mangrove. We often hear him blowing air out, while we are anchored in a bay at night, watching the stars...
Between December and April, there is the possibility of seeing short finned pilot whales (Globicephala melaena), which migrate along the African coast, in groups of upto several hundred individuals. When spooted, they can be observed for hours, as they seem to just float around, at a short distance from the boat.
From August to October, some whale watching can be done, but these encounters are pure luck! We can see humpback whales (Megaptera novaeangliae) and southern right whales (Eubalaena australis), which move along the African continent from the Antarctis, where they feed, to the more temperate waters, for giving birth.