Seagulls can drink salt water because of glands in their heads
If seagulls hadn't evolved to drink sea water, they would have died out or moved back to lake living long ago. Thankfully, evolution provided a special supraorbital gland that scrubs the salt from the seagull's blood and expels it as an ultra-salty liquid from the nostrils.
Given a choice, seagulls will drink fresh water over salt water because processing the salt is an energy-intensive process. But in a pinch, and when they're far out at sea, they can lower their beaks to the sea to quench their thirst.