what does the salt water refers to(指的是）?（In English）

what does the salt water refers to(指的是）?（In English）
We all know that dogs,pigs,and monkeys are very intelligent animals.But when it comes to underwater intelligence,1 no animal is smarter than the dolphin.No one knows for sure how smart dolphins are,but we do know they are fast learners.They can perform2 many amazing3 tricks,such as ringing bells with their snouts,shaking hands with their flippers,and taking bows.They can even act.In the movie Flipper,a dolphin was the main star!
　　Because most dolphins live in the ocean,you might think they’re fish.Actually,they are mammals,just like people.That means they grow hair,give milk to their young,and breathe air.Every so often,the dolphin must rise to the surface and breathe through its blowhole.Like land animals,dolphins can’t drink ___salt water___.They get most of their fresh water from their food.So even though dolphins live underwater,they survive4 just like desert5 animals,with no direct source6 of drinking water.