Some parts of the sea are saltier than others. The Atlantic Ocean for example has two huge deserts of extra salty water. To see which parts are saltier than the other, NASA's Aquarius instrument aboard Argentina's Aquarius/SAC-D spacecraft released the first year of data.
You can see it in the image above, red and orange areas are very salty, while green and blue areas are less so.
Wired has published a very nice feature on Kip Thorne and the science behind Chris Nolan's Interstellar. Kip Thorne is one of the world's most celebrated theoretical physicists. He and Nolan worked together to ensure depictions of scientific happenings in the film are as accurate as possible. Read more