For people with pools, this is more common than one would think. First off all, toddlers have no business being within 10 feet of a pool edge without a floating device and outside a guardians' arms. Even with a floatation device, they can hardly keep themselves upright.
This happened to my son as a 5 year old. We had neighbors over and he was an okay swimmer for his age, but not the fish he is now. He slipped down the slope toward the deep end of our pool and was underwater for less than a half-second, before I jumped in with my non-waterproof Fitbit, a set of Bluetooth headphones, and my non-water resistant cellphone.
All needed to be replaced within 6 months (except for my son, of course).