That's a great question. People can't drink sea water because we can't make urine that's saltier than sea water, so we lose more water to carry away the salt.

Marine mammals have to get water from what they eat or from the sea. Whales actually drink sea water. I suppose that other marine mammals do this too. They have very big kidneys that can get rid of the salt without losing a lot of water. They also don't sweat (one way we lose a lot of water) and they don't lose a lot of water when they breathe. People lose a lot of water just by breathing. There aren't too many Santa Barbara days when it's so cold you can see your breath, but you still breathe out a lot of water. (Why can you only see your breath on cold days?) Because the air at the surface of the ocean has a lot of water in it, marine mammals don't lose much water this way.

So why don't we have those big, hard-working kidneys so that we can drink sea water? Well, those kidneys take a whole lot of energy to work. During most of our evolutionary history, food was in short supply, so people who had energy-hogging kidneys would have been at a big disadvantage almost all the time.

I recommend the site of The Marine Mammal Center where you can read interesting information about mammals: here