The Emerald Sea (Mer d'Emeraude) is located north of the passage to Diego Suarez Bay.
It is one of the largest and most beautiful natural bays in the world.
It is an enclosed bay with ~20 Km boundary.
A visit to northernmost Madagascar should include a trip to the Emerald Sea.
The ideal time is during the rainy season from November to March, when the water is clearer and warmer.
Organised boat transfers leave from Ramena Beach or Port Jasmin in Diego Suarez.
Besides the transfer, organised tour operators usually also provide snorkelling equipment as well as lunch at a deserted island.
Local fishermen at Ramena Beach are willing to take passengers to the Emerald Sea for a much lower price.
Even from space the colours of the Emerald Sea are highly visible. The first metres of the bay are shallow turning the water emerald, thereafter the water gets a bit deeper turning the colour turquoise instead of blue black.
Several mostly deserted islands with great empty beaches frame the bay. Beside sunbathing and swimming, Emerald Sea is ideal for snorkelling, kitesurfing and windsurfing. It is also possible to stay overnight in one of the few hotels on the mainland next to the bay. Babaomby Island Lodge offers accommodation in large tents.
Reference: http://www.madacamp.com/Emerald_Sea