You are correct. I have seen alligators in drainage ditches. If you are in Florida and see any kind of water, you need to assume there is an alligator in it.
The only exceptions I can think of were the lakes they dug to get material to build overpasses. They were steep-walled reservoirs, and quite cold. I did a little snorkeling and speargunning without worrying. But, any drainsge ditch was suspect. I remember looking in one around a parking lot. ‘Gator was probably waiting for a cat or dog to take a drink.