Reading a children's book on penguins got me to thinking about animals that inhabit both land and sea during their lives. Penguins can dive over 1000 feet in frigid waters. They eat fish and krill under water, yet mate, incubate, and feed their young on land.
Penguins are birds. A reptile that lives in both land and ocean is the sea turtle. Frogs - which are amphibians - have life stages where they breathe under water (as tadpoles in ponds and streams) and later in life, breathe on land as full-fledged adults. Mammals that thrive on land and sea include seals and sea lions. There are some crabs - crustaceans - that fit into this category. It's not that these creatures are flexible or adaptable, but that their life cycles require both worlds.