Seals, whales, dolphins, manatees and hippos were all once land animals who moved into the water, and water worked its magic on their shapes. Proconsuls lost their fur, growing longer head hair so that toddler apes paddling around their mothers had something to grab onto if they got tired. Another reason our ancestors lost their fur is that fur does very little to keep an animal warm underwater, and wet fur is really chilly. However, a layer of subcutaneous fat, like the blubber of whales and other aquatic mammals, keeps the internal organs warm. Humans are the only primates who have such a layer of fat.
Female breasts are another effect of the water.
Baby apes cling to their's mother's fur to hold themselves up when nursing, but Pronconsul mothers soon had no fur. Protruding breasts gave the baby something to hold onto. Their breasts often sag pendulously to their waists. This may not look very attractive to us, but it is convenient for their babies. Not only that, but they hold their young to nurse them much the way we hold ours.