Meet the iSwimband: a smart wearable device that lets you know when someone is submerged dangerously long. It is useful for those who have small kids who play in the pool all the time. The system is available for non-swimmers and swimmers in form of a wristband and headband respectively. You are provided with an audible/visual alarm on your smartphone or tablet if a swimmer remains under water for too long.
The device has a range of up to 100 feet. You can monitor up to 8 iSwimbands per iOS device. Sets you back under $80 at this time.