It's a small detail that I think you can only find before entering the water, but Mari gets a little upset with Omori if he runs, and then gets more so when he keeps running. After seeing the second bit of text, your ability to run is locked for the rest of the section.
I get the feeling Mari really raising her voice at Sunny was something that pretty much never happened. So whenever she did, he would absolutely listen. Especially considering what happened the last time she yelled at him and he continued to disobey her.