Poolguard PGRM-2 In-Ground Pool Alarm
If you need a pool alarm that cannot be deactivated once it is set in place, than the Poolguard PGRM-2 In-Ground Pool Alarm is the one for you. It must be physically removed from the pool and put into sleep mode to deactivate, which means it is tamper proof even to the point of being stolen, unless you actually know how to deactivate it. It runs on a 9-volt battery, has a 200-foot remote and automatically resets when placed back into the pool. It would have scored higher on the list, but deactivation sometimes causes false alarms, and if you are trying to take a midnight swim in a suburban neighborhood, that will cause problems.