Though not an amphibian, Godzilla has an amphibious lifestyle. Godzilla is described in the original film by Doctor Yamane as a transitional form between aquatic and terrestrial vertebrates.
Godzilla is almost certainly able to somehow extract oxygen from water, occasionally remaining in the ocean depths for periods of months or years, though there has been no official explanation or reference to this ability aside from the fact that he possesses gills.
Underwater, he feasts on mammalian meat of whales, and schools of fish. Godzilla lies down to sleep in the same manner as humans. At times he can be found sleeping inside mountains or lakes.
Capable of marching on the sea floor or swimming by undulating his tail like a crocodile, Godzilla is displayed as being able to breathe underwater (occasionally hibernating in the ocean depths between movies), and being submerged apparently does not impede his atomic ray.
He engaged opponents in the sea on multiple occasions, fighting King Ghidorah, Ebirah, Battra, Biollante and Mothra either beneath or on the surface of the waves.