Your body needs a small amount of salt (electrolytes) to operate, and the more you sweat and get dehydrated the more of these salts you lose.

So drinking fluid with an appropriate amount of electrolytes helps hydrate you.

The problem with ocean water is that it has waaaayyyy too much salt and your body reacts to this by trying to get it out of your system via your urine. So you use more water to get it out of your system that you are taking in.