Folk, as has been said many times....it IS a garter snake! Four feet is about the maximum size, not 5 feet.
It was under the pool because it was cool, moist and it attracted its favorite foods...crickets, earthworms, frogs, and SLUGS! They will eat small fish and are really good swimmers.
There are many different species of garter snakes and they are quite variable even within the species. Eastern garter snakes are everything from dusky brown to red sides to bright turquoise blue. In the Fort Lauderdale area is a population of brilliant blue eastern garters. I have collected quite a few. I wish I had photos of them.
The long thin members of Thamnophis with the stripes are the ribbon snakes, Not garter snakes.
If you garden, leave the garter alone. It offers no danger to your chicken eggs (too big) and it will hunt pest that eat your plants. Anything that will eat slugs is welcome at my house.