He has paused in the water at the sound of a whistle and is watching for the hand signal that will tell him what to do next.
This control of a dog from a distance is called handling in the retriever world, and it's a sign of a well trained and fairly advanced dog, especially when they will turn and wait while treading water. The addition of water increases the difficulty of almost all phases of training and performance for these dogs, due to either the excitement of working in the water (which they love) or the increased strength and stamina required, which tires them out more quickly.
This dog is older, as can be seen by his whitening face. He is a seasoned and experienced dog, thoroughly familiar with all phases of fetch work on land and in the water and, as can be readily seen, he leaves a broad and choppy wake even in his later years.