How do the functions of the forelimbs differ among these vertebrates? (what are each used for? What does a seal use its flipper for?

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The physically alike but functionally diverse structures that share a common predecessor are called homologous structures. The forelimbs are an example of a homologous structure.

The function of forelimbs differs in the organism as:

Thus, forelimbs differs in function.

To learn more about homologous structure follow the link:

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Homologous organs can be described as structures having the same internal organization.

The forelimbs of many vertebrates have the same arrangement of bones. Some of these vertebrates having similar organisation of forelimbs are bird wings, bat wings, flippers, and human arms and legs.

Although the function of these limbs yet they have similar internal organisation.

A bird's wing helps the bird in flying. Bat wing helps the bat to fly. Human arm and legs are used for movement. Flippers help in movement in water.