I sent my 6YO to Valley Mill for the summer. He came into the program as not a very strong swimmer and left the program being able to swim across the length of the pool on his own. We now have him in swim instruction to develop his strokes.
He took formal swim instruction at the camp at least once a week and everyday there was a "free swim". I visited on Parents' day when there was a swim instruction and a free swim. I observed that they had the kids separated according to ability and worked with them to develop their skills. There were at least two counselors for each group of no more than 10 kids. During the free swim, there were multiple counselors in the pool with the kids. The area of the pool where DC stayed was the shallow part. Stronger swimmers, were in the deeper part of the pool, but still supervised and I don't think there was really a deep end.
We are really happy we sent DC to a camp with a daily swim program. We had him in swim lessons for years before that, but believe that daily exposure to the pool and swimming helped develop his skills and confidence.