A boy in swimming trunks wades out into the water.
The young swimmer wades out into the water at the start of his team practice.
Just because a boy is swimming does not mean he is at his team practice.
The boy might not be young, he might not be a swimmer since he is only wading, and he may not be going to team practice.
There are many time frames that a swimmer can wade out into water so one cannot assume with certainty thta it is at the start of team practice.