The younger Cousteau and a team of researchers went under water at around 11 a.m. EST Sunday and are expected to live aboard the Aquarius Reef Base, an underwater habitat about 63 feet below the surface.
The expedition, dubbed Mission 31, will include the testing of new technologies and the studying of the effects of climate change on underwater sea life like corals and sponges.
"The overarching theme for Mission 31 is the human-ocean connection within the lens of exploration and discovery," Cousteau said in a statement.
The mission is being webcast live on the Mission 31 website. Cousteau is also working on a book documenting the experience.