Penguins drink salt water. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have evolved into flippers. They swam on the surface using mainly their feet, but the wings were – as opposed to most other diving birds (both living and extinct) already adapting to underwater locomotion.
They can drink salt water because their supraorbital gland filters excess salt from the bloodstream. The salt is excreted in a concentrated fluid from the nasal passages. The great auk of the Northern Hemisphere, now extinct, was superficially similar to penguins, and the word penguin was originally used for that bird, centuries ago. They are only distantly related to the penguins, but are an example of convergent evolution.