What is an underwater iguana?

The Marine Iguana (Amblyrhyncus cristatus) is a species of iguana endemic to the Galapagos Islands (off of Ecuador). They populate the rocky seashores of the island, and go underwater for the purpose of eating algae off the rocks on the seafloor. Because of their highly specific diet and habitat, very few of them have been maintained in captivity outside of the island (and even then with little success).