We live on a spring-fed lake, so the water is very clean and clear. Ozzie and Belle were in the water at 10 weeks. I put them in the day we brought them home. They were in the lake before they were in the house! It was too late at night when we got home with Bandit, so I waited until the next morning. Bandit was 16 weeks when we got him.
We were fortunate that we got all 3 Newfs when it was warm enough for me to get in the water. I took each one down to the lake and swam with them. Belle didn't even wait for me to get in the water! She knew what she was supposed to do as soon as I set her down. With Ozzie and Bandit, I held them and walked out a few steps and then gently lowered them into the water, all the time making sure they were not afraid. As soon as their paws hit the water, they both started "swimming". I lowered them down until they were actually swimming and walked along side them until they got back to shore (about 10 feet). After a few times, I didn't even have to get in with them.