Dogs, cats, and virtually all animals except people can swim naturally, may not have a strong instinctive fear of water and we don't think about animals in nature dying by accidental drowning. Escape from a swimming pool can be just about impossible for a small dog or cat unless you provide some sort of ramp and make sure they know how to use it. It must have been horrible for those people, who really cared about their pets, to find them that way. There are other risks that come with hot weather. Overheating is the most dangerous.
Most people know it can be fatal to leave a pet in a closed car on a warm day.
Even in the brief time it takes to run into the store for something, a dog's temperature can increase dramatically. Over heating can also be caused by over exertion, and several years ago I even had a canine patient who had just been left in a shaded back yard die from heat stroke. Cheat grass is the most common summertime problem. If you don't know what cheat grass is, go for a ride out in the country around here until you see some brownish - reddish weed patches alongside the road. Take a walk through one of those weed patches, and when you feel a bunch of little prickly things sticking through your socks, that's cheat grass.