You need a wave/splash detector to detect dogs, small children, drunks, and errant waterfowl. My intent was to deter waterfowl, we have Canadian Geese, but if it can detect a goose in the water, it can detect a dog.
I soldered a vibration sensor in the place of a reed switch on a Iris V1 sensor (which come with cute little waterproof gaskets!), and then put the sensor inside the hull of a toy plastic boat. The "alarm" is deactivated by pushing a button mounted by the back door, which gives one 5 minutes to enter to pool and remove the toy boat from the water. When one is done swimming, one returns the toy boat to the water, and the vibration of doing so re-activates the alarm after another 5 minute delay. You may need to ballast your boat to avoid picking up ripples made by wind, as this scheme is useless without a klaxon loud enough to wake the dead, so you want to only pick up actual waves and splashing from a living creature in the water.
