Coarse Underwater Proximity Sensor

I am working on a project that requires detecting when a person in a swimming pool comes within a couple feet of my device. I don't need to know the distance the person is away - just that they came within a couple feet. I have read several posts regarding use of the JSN-SR04T for underwater distance sensing and it seems like results are mixed. Can anyone weigh in on the utility of that sensor if it's being used as a binary "you've been detected" or "you haven't been detected" mode?

Also, does anyone know how the sensor behaves if you are very close to it underwater? I appreciated that speed of sound underwater is much faster than air and that this might pose a limitation for calculating actual distance as signal return time would be very short. Just wondering if it would still work to detect presence of person very close.