Drinking water when swimming?

Yes, we need to drink water while swimming because we loose it by that time. When you loose water you need to replace it somehow so by drinking water you fulfill your needs of it while swimming. I also drink it when I go to the swimming pool. I feel thirsty if I don't do it.

sometimes I drink it because when our mouth is open, a litle water is enter to our mouth

Yes, Ebrahimebram, I do drink water while swimming. As a matter of fact I keep a bottle of drinking water at the edge of the the pool while swimming and I will drink from it whenever I feel thirsty. Even though we are swimming in water and our body is wet we are still sweating and losing water from the inside of us that needs to be replenished.