Why is a whale considered a mammal even though it lives in water like a fish?

What the question is really asking

You are being asked to classify an animal by its body traits, not by its habitat. Living in water does not automatically make an animal a fish.

Mammal features whales have

Whales match the main characteristics used to identify mammals:

• They breathe air using lungs, so they must surface to inhale.
• They are warm-blooded (their body temperature stays fairly constant).
• They give birth to live young (they do not lay eggs like most fish).
• They produce milk, and whale calves nurse from their mothers.

How this differs from fish

Most fish:

• breathe using gills (they do not need to surface for air),
• are cold-blooded (body temperature changes with the water),
• typically lay eggs,
• do not feed their young with milk.

So why whales are mammals

Even though whales are adapted for swimming and live in the ocean, their breathing, body temperature control, and way of reproducing and caring for young fit the definition of a mammal.