These seem to be (based on two) lifeguard rescue videos. 30 seconds of lots of kids in a pool, lifeguard jumps in and swims to a person you were not able to spot needed help, and then they help that person back up safely.

By "drowning" they mean "displaying the instinctive responses to perceived difficulty maintaining breathing in the water"

I haven't watched all of them. So far no one has drowned. For a variety of ethical and legal reasons I very much doubt any reputable organisation would put a video of a fatal drowning on their site.

In the 5 or so videos I watched they were all rescued!

The app seems to be timing your response relative to response of a lifeguard: negative seconds if you noticed before a lifeguard jumps into the water, otherwise positive.

They don't drown/die. There is an instinctive response to not being able to keep oneself over water that has certain "look". The videos show busy pools and you basically try to find Waldo before the lifeguard pulls them out.