I think you need to start off with the recognition that no matter what you do, there's going to be no guarantee that this is going to solve every specific problem that arises with a given child in a given pool. Since there is no guarantee, the best we can to is to provide layers of protection.

Parents should know that adult supervision is the only way to prevent accidents. They should maintain constant contact with children whenever they are near, or could get near, any body of water.

Unfortunately, most accidents occur when there is a lapse in supervision, even for a short time. The Association of Pool and Spa Professional recommends the following safety measures for your pool, spa, or hot tub.
 
1. Fencing to isolate the swimming pool at a minimum of four feet high.
 
2. Safety Covers can be used during the off-season to keep kids out of the pool. However, you should know that a small child can fall on a safety cover and still drown.
 
3. Door exit alarms will warn parents when a child opens a door to the pool.
 
4. Self-closing/self-latching devices for doors and windows can limit access to pools and spas.
 

5. Fence gate closer and latch can limit access to pools and spas
 
6. Fence gate alarms sound when a fence gate is open.
 
7. Infrared detectors are wireless detection alarms that sound when the area around the pool perimeter is entered.
 
8. Pool Alarms sound when there is accidental or unauthorized entrance into the water.
 
9. Child Alarms can be attached to children and indicates when that child has entered the pool.
 
10. Rope and Float lines should be placed across the pool, alerting swimmers to the separation of the deep end from the shallow end of the pool.
 
11. Rescue equipment (life ring, shepherd's hook) should be placed near the pool in an easily accessible spot and should be kept in good condition. These can be used to pull someone in trouble to safety.
 
12. Posted emergency information should be posted near the pool or spa. Examples include: CPR instructions, warning signs, emergency phone numbers.
 
13. Outside telephones means parents don't have to leave children unattended while they answer the phone. Also, it's a good idea to have one handy to summon help, if needed.