Bipedalism on the grasslands would have slowed the species down, as species that run on four legs are much faster than species that run on two. Probably the best way to examine this theory is to ask what specific features an aquatic animals would be expected to have. Strangely, we actually have more hair follicles than our cousin, the chimpanzee. The difference is that human hair is very fine and short, giving us a hairless appearance. Human babies are born fat, whereas all other primate babies are born lean.
Human babies can swim from the moment of birth.
Other primate babies cannot. Human babies not only float but also, after being born submerged, can swim under their own power, holding their breath until reaching the surface. In the water, the human baby is not helpless. From the moment of birth on, the human baby can swim alongside its mother. Fat is a characteristic of marine mammals.