Pool Sensor

Just got an email from SS reminding us it's National Security Month (hallmark holiday? :-)  and the idea of putting an outdoor camera to monitor the pool and area around it. Nice idea to sell cameras but 1. it takes 10 seconds + for the camera to start up for live streaming and who would trust the motion detection to prevent a drowning?  Sure, an outdoor camera is nice but, as someone before me suggested, a "pool sensor" that would sense water disturbance at set times or a simple way to turn on and off would be much more useful and effective, at least in theory.  I don't even know if something like that even exists but those that have pools would surely want one...if it worked and existed. For record, we have no pool or would want one.