The aquatic ape hypothesis (AAH), also referred to as aquatic ape theory (AAT) or the waterside hypothesis of human evolution, postulates that the ancestors of modern humans took a divergent evolutionary pathway from the other great apes by becoming adapted to a more aquatic habitat.
In short:
Much more than other primates, man has several features that are seen more often in aquatic than terrestrial mammals: nakedness, thick subcutaneous fat-layer, stretched hindlimbs, voluntary respiration, dilute urine etc. The Aquatic Ape Theory states that our ancestors once spent a significant part of their life in water. Presumably, early apes were plant and fruit eaters in tropical forests. Early hominids also ate aquatic food; at first mainly weeds and tubers, later sea shore animals, especially shellfish. With the Pleistocene cooling, our ancestors returned to land and became bipedal omnivores and scavengers and later hunters of coastal and riverside animals.
Source: https://pubmed.ncbi.nlm.nih.gov/3923303/#:~:text=The%20Aquatic%20Ape%20Theory%20states,sea%20shore%20animals%2C%20especially%20shellfish.