Good post on why they don't catch on
There's no unassailable reason why primates could not have returned to the sea and produced an aquatic variation. Some cryptozoologists think there might be an aquatic primate behind some "mer-creature" tales. But the idea that human ancestryy went through a phase dominated by water living - the "aquatic ape theory" - doesn't get much traction among anthropologists (or even cryptozoologists, for that matter.).
In this blog post, John Hawks explains this. It's not one of those horrible academic conspiracies that pseudoscientists like to claim (and which do, on rare occasions, exist de facto if not de jure). The problem is that, even if you argue some characteristics like hairlessness were an adaptation to a water environment, you have to explain why those same characteristics were evolutionary advantages when the species returned to the land. In other words each characteristic must be explained twice, and make sense both times. In other other words, I don't have much use for this theory, which is often buttressed by questionable assertions about individual features and is undercut by, among other things, a complete lack of fossil evidence.