Cat's kidneys are so efficient they can rehydrate by drinking sea water. Is it true?
Even if it's true, their kidneys won't last long on sea water.
Sea water is not able to rehydrate anything due to the high salt content. That's why people who are lost at sea cannot drink sea water. No, I don't know where you heard this but I doubt that it's true.
It's unsafe to give them tap water if it has high content of salt so I really doubt sea water is ok for them.
They can due to their efficient kidneys. They can even survive on a diet of all meat and derive all the water they need from the meat.
What does eating meat have to do with drinking sea water?