> What do people usually expect drowning to look like?

Head erect, more splashing, perhaps a cry for help, and above all, someone who thinks and acts like a human being. You can't tell from the site, but a drowning swimmer is a wild animal. If you have to approach at all, approach with extreme caution and with something to extend your reach.

It may be you aren't surprised because the premise of the site is to identify people who are drowning. You know it'll be happening within a minute or so. In real life, it'll happen with small probability in the next few hours. It's a lot harder to spot under those circumstances.