Swimming Pool Alarms

Swimming pool alarms are required by law in many areas and a good idea even if they aren't. Along with sturdy fences, they ensure kids can't get into your pool when you're not around, which could lead to accidents you would be liable for. Though the legal requirements vary from city to city as far as pool alarm necessities go, the Consumer Product Safety Commission has a few basic recommendations:

Pool alarms should be loud (minimum of 85 decibels) and distinguishable from other common household sounds such as telephones and smoke alarms.
All the gates leading to the pool should have alarms that go off for at least 30 seconds when the gate is opened. (There should be a deactivation switch for adults that turns off the alarm for 15 seconds--this should be at least 54 inches above the ground so kids can't reach it.)
The alarm should automatically reset.

Besides swimming pool alarms, you should have a sturdy fence and a pool safety cover (regular pool covers are only designed to keep out debris and keep in the heat--pool safety covers do these things but can also support the weight of anyone that walks out onto the pool cover).

Source: Sunset's Swimming Pools and Spas