According to galttech.com, approximately 300 children under the age of 5 die from drowning in the U.S. each year. One of the best ways to prevent this from happening is by installing a pool alarm system, which---in some instances---may also help ward off fence hoppers and pool trespassers. Pool alarms differ in their designs, and may feature above- or below-waterline disruption sensors. In some cases, the sensor may even be incorporated into a bracelet that a child must wear.
MG International Sensor Espio
According to goodhousekeeping.com, one of the greatest features of the Espio is that you can switch it from "armed" mode (wherein the alarm will sound if the surface of the water is broken) to "swim mode" via a wireless remote control. Additionally, if you want to change the alarm to "swim" mode from inside the pool, you will need to use a magnetic key, which makes it difficult for children to bypass the alarm. Another nice feature of the Espio is that it automatically arms when the pool is not in use.
Goodhousekeeping.com recommends the PGRM-2 as one of the best and least expensive pool alarms on the market. Unlike the Espio, with the PGRM-2 there is no "swim" mode, which means you have to physically pull out the alarm's sensor when people are swimming, and put it back in afterward to arm it. The alarm becomes armed automatically once it is in the water. One drawback of the PGRM-2 according to the above review, however, is that children can easily bypass the alarm by holding down the disarming button and removing the unit from the pool.
RJE Technologies AquaGuard
Similar to the Espio, the AquaGuard features a "swim mode" and will automatically rearm after people have left the water. However, this pool alarm does not use a remote or magnetic key, but instead relies on a pass code for manual arming and disarming.
First Alert A100 Safety Band
Unlike the previous pool alarm systems that rely on in-water sensors, the First Alert A100 Aqua Alert Water Activated Safety Band is an all-in-one unit that a child wears around her wrist. According to galttech.com, the Safety Band works by producing a 110-decibel alarm when it---and the child wearing it---enters water, be it a pool, pond, river or lake. However, one of the downsides of a system built on using the bands is that your child must put the band on--and keep it on--whenever open water is close by, which can be a hassle.
Safety Turtle Base Station / Wristband Alarm
Unlike the above bands, which individually sound off when exposed to water, the Safety Turtle wristbands signal a base station, according to galttech.com. This station includes one bracelet and is typically kept inside where parents can monitor it. Additional wristbands for the Safety Turtle base station are also available.