The marine iguana (Amblyrynchus cristatus) is the world's only seagoing lizard. Like other marine reptiles (except some sea snakes) it still must return to land to breed. However, it seems better adapted to life in the water. It swims gracefully with sinuous movements of its long tail, which is strong enough to propel it through pounding waves. Only the big males need to take this riskto get to where there's enough food for them. Their main diet is low-growing algae that does best in cool water. Females and young iguanas feed on the shore and rarely venture into the ocean. Little is known about the risks from predators. It is assumed that some of the iguanas fall victim to sharks, but this is not documented. Sealions leave them alone, except for boisterous pups who like to harass a helpless iguana by pulling its tail!
Marine iguanas are common on land, but it's not on every time you snorkel that you will see a marine iguana underwater. So you're very lucky to see an iguana feeding. I had done quite a few snorkels around promising areas until I saw this one just by chance near Espinoza Point on Fernandina Island.
This iguana is feeding at low tide, just beyond reach of the surf. Notice how close its mouth is to the low-growing algae. (Taken at Espinoza Point on Fernandina Island.)