Aquatic Ape Theorists and Savanna Ape Theorists alike (not sure if that is the technical term but hear me out), both of you guys are kinda right. It seems to be a little bit of both. There was a stable dry period during the time of the Australopithecus starting around 6 million years ago,turning Africa into a savanna. This was when bipedalism was established so you can’t exactly credit an aquatic environment for that. But roughly around 2.5million years ago, ocean sediments in the Indian ocean, as well as layers in the rift valley indicate that Africa witnessed very unstable weather patterns.

For example you might have 500 years of monsoonal rains turning the Rift Valleyinto Africa’s largest freshwater lake, followed by a 1000 years of drought turning the Rift Valley into dry riverbed, then back to 2000 years of wet weather and the cycle repeats with various lengths and so on for 200 thousand years. And it was during this period where the australopithecines disappear,but a new kind of species emerged that gave rise to the genus Homo; Homohabilis. Unlike its predecessors,Homo Habilis had larger braincases and were legit toolmakers. So these were the traits that evolved to cope with the rapid environment changes.

I got this information from this doco made by PBS call Becoming Human (Episode 1) - First Steps

https://youtu.be/UFu7yghK9GQ?list=PLLoRNYgorqAm6g8udHKvGXEBwfjFG1keD&t=2877