Penguins swim and feed in salty water. If they drink this water, they will also take in the salt. But too much salt is very damaging because cells loose water due to osmosis. That is why penguins have got salt glands in their forehead region which transport the excess salt out of the penguin's body. The salt is filtrated out of the blood because of a counter flow principle, so sodium and chloride ions (= cooking salt) get in specific interspaces. This interspaces flow into a central canal which leads to the nostrils. There the salt is excreted eventually.
Biologie by Campbell, Neil A. and Reece, Jane B., 8th edition (German book)
Tierphysiologie by Roger Eckert, 4th Edition (German book)