They are referring to the "blacks cannot swim" stereotype. Black kids are more likely to drown in pools and in these videos they appear more as well (see https://www.abc10.com/article/news/community/race-and-cultur...).
You ever swim with clothes on? Like a pair of jeans? It's way harder, way more energy gets expended to stay above water. Even with knowing how to swim, having a reduced ability to float can be incredibly dangerous.
I did find a couple where you could tell which kid was going to be in trouble long before it'd let you click them, which I found interesting.