The real issue with these is they are a last line of defense, and your child is already in the water, and "if" it goes off, and "if" you are where you can hear it, and "if you can get to them in time.

In my opinion these are a third layer.
Start with the house doors, then a fence, and if you are still not comfortable then sonic alarms.

There are also laser beam style alarms that you can set up a perimeter around the pool, so you can be alerted when someone gets near the water, but before they are in it.

Swim lessons and supervision are primary.

Failures to listen (kids push to test Mom's limits) MUST be met with strong reactions from the supervising adult.

Fences need self closing doors with latches that the kids can't reach.

Alarms are OK but like any safety device, it needs to be used or it's worthless. Many communities require them to pass inspection.

Scott