Taking the lifeguard job means you might have to blow the whistle on your own friends. In that chair, fair beats friendly.

Taking a job with authority in it means accepting that the role sometimes outranks the friendship, which is uncomfortable because your friends will not always see it that way. Being fair to everyone, including strangers who do not know you, is the only thing that makes the chair mean anything. A friendly chair is no use to the person at the bottom of the pool. The job is the point, not the popularity.

Tell your friends in advance how you will handle it, so the first whistle is not a surprise. Then treat them exactly as you would treat anybody else.