@Dirk you mean like the pool alarm people? They collect a cool $200-300 on every new pool installation for surface alarms and door chimes that are needed to pass inspection. These things are then immediately disabled / thrown away because they do nothing but go off for no reason at all hours of the day and night. It's like a new-pool-owner tax that Lifebuoy/Poolguard/etc. are happy to collect.
What he means is they work TOO well.... a strong breeze sets them off. Leaves dropping in the pool set them off. They annoy people to death so they remove them pronto.
I'm a big fan of automatic safety covers..... a flip of the key switch and the pool is open or closed. Of course they're a tad pricey. Fencing and door locks are basic and don't usually break the bank.