I discovered that many mobs will slow down or stop near the end of a full 8-block water stream, specifically in the last three blocks. I tested it with villagers, evokers, husks, skeletons, and creepers. The undead mobs don't stop, but they slow down significantly. It seems to have something to do with the splashing up and down, since the undead mobs no longer swim to the top of water, but sink instead. I waited for ten minutes with the creepers and the only ones to fall were due to collisions with other creepers.