Plage de Montjoly is the long sandy beach just east of Cayenne, the closest swimmable beach to the city. The water is brown — the Amazon dumps so much silt into the Atlantic that the entire Guianan coast looks more like coffee than ocean — but it's safe to swim, and the beach is a popular weekend retreat for cayennais.
The real draw is sea turtles. From April to July, leatherback, green and olive ridley turtles come ashore at night to nest, and conservation NGOs run guided turtle-watching tours from the beach. By day, walk the long flat sand and watch the frigatebirds dive offshore.