Most marine lizard (living)

The most marine lizard alive today is the Galápagos marine iguana (Amblyrhynchus cristatus), endemic to the Galápagos Islands, a volcanic equatorial archipelago belonging to Ecuador in the Pacific Ocean. Uniquely sea-going for an extant lizard, this species habitually forages off the islands' coasts for the abundant red and green algae existing in the intertidal and subtidal zones that constitute almost its entire diet. It can stay underwater for up to one hour before needing to surface, and can dive up to 30 m (100 ft) deep. When swimming, females and juveniles stay close to shore, but the much larger adult males will swim farther out to sea in search of algae and will even travel up to 65 km (40 mi) between islands. It has evolved special salt-excreting glands inside its nasal cavity that remove from its blood the salt in the seawater that is ingested while underwater, and then effectively "sneeze" it out.

Back in the Mesozoic Era, there were large (sometimes extremely large) sea lizards known as mosasaurs, closely related to today's terrestrial and freshwater monitor lizards or varanids, but which unlike today's marine iguana were exclusively marine and had even evolved flippers and finned tails, whereas the marine iguana still possesses clawed feet and a non-finned tail and spends a large part of its life on the shore.

It is nowadays thought that the marine iguana and the Galápagos land iguana (lately divided taxonomically into three closely related species housed in the genus Conolophus) split from a common ancestor about 4.5 million years ago, after the latter had reached the Galápagos Islands from Central or South America by rafting. This would have been not long after those islands that still exist today had themselves come into existence. Moreover, being volcanic in origin, they would have possessed only sparse vegetation back then (some are still quite arid today). Consequently, it seems plausible that, seeking additional food to supplement their diet, some of these sizeable iguanas ventured into the sea where they discovered a rich supply of marine algae, which they subsequently evolved to graze upon. This highly specialized, singular scenario may in turn explain why there are no other habitually marine lizards alive today elsewhere.

That said, there are some other species, including today's largest lizard, the Komodo dragon (Varanus komodoensis) of Indonesia, that are occasionally seen swimming in the sea close to shore but only for brief spells and, unlike the marine iguana, they are not equipped biologically to dive or travel long distances out to sea.