A voice monitor, similar to the kind new parents use when they have a newborn baby, is a great idea for your swimming pool area. Strategically place several monitors around the area and make sure they are waterproof. Check the batteries in them regularly to assure they are working properly.
Voice monitors can alert you to the sound of voices in the swimming pool area.
Turn the monitors on anytime young children are present in the home or playing outside, and listen for any activity inside of the pool safety fence.