Explain the evolution of the marine iguana from land dwelling to water dwelling?

The marine iguana is not a water dweller. It swims and dives in order to feed on algae. It is a close relative of the green iguana, which is also a good swimmer. Since food is not abundant on volcanic islands, the iguanas that reached the Galapagos islands began feeding on marine algae. Because the water is cold, marine iguanas must warm up by basking and take short dives to feed. Prolonged diving would lead to a fatal dip in the body temperature of the iguanas.