I have little experience with pool alarms of the sort you mean -- they are not used commercially -- but lots of experience with other times of alarms, and sensors . . . and there's a fundamental problem with almost all alarm systems:
  • If an alarm systems is sensitive enough so it never misses a valid TRUE alarm (like a kid falling in), then it is also sensitive enough so that it will have many false alarms!
  • Conversely, if an alarm is set to avoid all false alarms, then it is also set to MISS some TRUE alarms!
The only way I've seen around this is to combine an alarm with a camera, so you can easily check each alarm, and then reset it, if it is not a true alarm. I spent a few minutes with Google and Amazon, and looked at some of the "top 10" lists . . . and didn't see anything I'd recommend.

What might work well is a conventional alarm system, with a motion detector on the pool deck, a camera on the pool deck, and gate alarms. I personally use the SimpliSafe system (https://simplisafe.com/) and could set one up -- for myself -- if I was trying to do what I described. On the other hand, I know the conventional monitored alarm my uncle has on his property would be a total pain set up that way.

Bottom line?

I don't think there is any 'works out of the box' pool alarm solution that is better than mediocre.

Sorry!