Why do fish, seals, whales have a similar body shape?

The similarity of the structure of unrelated forms (for example, fish and mammals) is called convergence.
This is due to the similar environmental conditions in which such organisms live.

Fish, seals and whales live in the aquatic environment. In this regard, they have developed adaptations to it: a streamlined body shape, the transformation of mammalian limbs into fins, and in whales and seals, hair has almost or completely disappeared. All this contributes to a more efficient existence in water, therefore, such adaptations independently appeared in different groups.