Employ an Immersion Alarm
An immersion alarm is a device that you place on the edge of the pool, submerging one part of it. Its underwater sensor will detect if a mass falls into the water and set off a loud alarm to alert anyone in the surrounding area.
This option, however, does come with downsides: strong winds or the movement of certain objects, like balloons, can trigger the device, stressing parents out with false alarms!
Wear a Bracelet
You can also purchase bracelets that, when submerged, transmit a signal to a base unit placed wherever you want (in the house or on the patio, for example). The base will then set off a loud alarm.
All you need to do is have those children who are at particular risk of drowning wear the bracelet when playing in the yard. Several very pretty models are available that they will be thrilled to wear! Plus, it is almost impossible for the kids to remove them themselves because a special key is required as well as a certain level of dexterity.
Of course, this bracelet system is only effective under one condition: remembering to put the bracelet back on the child after every swim session.
But just a tip: once triggered, the alarm can sound for quite a long time. So, if you want to test if the bracelet is working properly, do it indoors so as not to alert the entire neighbourhood.
Use a Locking Pool Ladder
You need to be aware that pool ladders are not all created equal regarding their safety if you have an above-ground pool. For example, some can be locked up with a gate that encases the steps, rendering them almost inaccessible. If you own a regular ladder, it is recommended that you remove it after swimming.