Children should always be supervised when they are in the swimming pool area — regardless of whether or not they are actually in the pool. This is because even when they are wearing flotation devices, children can drown.
Adding protective measures, such as swimming pool fencing, does not help if you are not paying attention to your child when they are permitted to be the swimming pool area.
Close, detailed supervision should be available for every moment your child is swimming. If there are multiple children present, there should be more than one adult supervising. Do not encourage rough play in the swimming pool and go over pool area rules before allowing anyone to swim.