Zanibar- there are more than thirty beautiful beaches in this island. The excellent beaches of Zanzibar lies along the east coast of Nungawi, just a few distance away from the northern tip of the island. Dubbed as one of the worlds sexiest beaches of 2008.
Best month to Visit: July through March (February and March are the warmest months)

Beau Vallon, Northern Mahe – Seychelles
White and soft sands are expected during the northwest monsoon from November to April. Famous for its large beach, shaded with large full-grown tress and palms. Known for its cool and crystal clear water.

Mauritius
An interesting beach one must be in. Consider to be as one of the Africa’s most exotic beaches. Actually, Mauritius has 13 best beaches. If you wish to be here, take into consideration that December to April are generally hot and humid while June to November is cooler, drier months.

Jeffrey’s Bay
Experience the perfect waves of South Africa’s Surf Capital. One of South Africa’s excellent beaches. Famous for seafood, water sports and venture activities. Jeffrey’s Bay is also the home of Billabong Pro WCT surfing event.

Cape Town
Numbers of beaches can be found in this place. Clifton Beach might be the most popular but too many beaches still to be considered as excellent and amazing, few of them are Sandy Beach, Boulders Beach, Kalk Bay and more. Cape Town is not an ordinary place; it is a jewel and precious area in the world.

Mozambique
It is a 2500km coastline with white fine sand and crystal clear, tropical warm seawater that you might compare to paradise. Mozambique beaches hold an extraordinary beauty and fast becoming as one of Southern Africa’s beach location.

Lumo Island
Do you know that Lumo is the oldest living settlement in Kenya? Yes it is, and definitely it is amazingly surrounded by rich marine life. In this spot, one could have a most memorable diving experience thru the captivating coral reefs of the Indian Ocean. And even try to travel around by the traditional Dhow vessels that might add your momentous experience.

Diani Beach
In Kenya, just 40 kilometers away from Mombasa, Diani Beach grandly spread ten kilometers of white sand. Behold the beauty of sparkling, blue waters of the Indian Ocean and witness the alluring view of Diani Beach, known as one of the most beautiful beaches in the world.

Watamu Beach
Watamu Beach in Kenya has been voted as one of the top ten beaches in the world. Offers scuba diving, snorkeling and deep-sea fishing. The presence of the tidal creek is to allow the viewers to have an opportunity for bird watching.

Saadani
In Tanzania, Saadani is exceptional for its unusual combination of beach, river and bush.
Experience the breathtaking scenery of those hippos, elephants, crocodiles, flamingoes and some bird species.

Quirimbas
Quirimbas Archipelago in Mozambique extends a hundred kilometers of coast, too near to the Tanzanian border. Composed of 32 stunning coral islands, unexplored and obviously never been developed. This island resembles the Maldives twenty years ago. 

Bazaruto
Bazaruto Archipelago is composed of well-conserved islands. Surrounded with white sand beaches and guarded by crystal clear waters. Filled with some of Africa’s most unspoiled coral reefs and can be explored through snorkeling or scuba diving.

References:

http://www.threebestbeaches.com/africa/
http://www.southafricalogue.com/travel-tips/top-ten-south-african-beaches.html
http://www.rhinoafrica.com/destinations/mozambique/
http://www.enchanting-africa.com/EA-recom-beach-more.htm