Why are polar bears and penguins good swimmers?

Penguins evolved to be good swimmers. They are adapted to swimming. That is their way of life. The way they swim is the same way birds fly. So, in a way, it is not much of a leap to change from flying to swimming.

Polar bears evolved from brown bears. Brown bears are good swimmers, but polar bears are even better swimmers. Most mammals are good swimmers. Elephants can swim. Even house cats, which don't like water are good swimmers. So are dogs. Even cows and horses can swim. So can mice. They are naturally good at swimming, because they have bodies that are lighter than water so they can float without needing to generate lift. The swimming stroke they all use, namely doggy paddle, is simply the way they move while on land. That is why even a kitten that has never swam in its entire life knows instinctively how to swim. Polar bears have also learned to swim long distances because during warm interglacial periods, the amount of sea ice is reduced and they need to go from land to sea ice to hunt seals. The bears that can swim better will survive while those that cannot will perish.

Because their prey is usually in the water

Are Polar Bears Good Swimmers

Because the ones that couldn't swim well drowned or starved.

They evolved to be in that environment.

Evolution. Over the generations with where they migrated to, they adapted to the prevailing conditions in which good swimmers survived while not so good swimmers didn't.