Shark in the Water is a term used to convey that one is menstruating, and thus is dangerous; or to say that the "waters" are "off limits" for that week.
The water that hardens the egg softens the potato is a Russian saying that means everyone reacts to stressfulcircumstancesdifferently. It is often used in the context of trauma.
"Rachel's mom just died and a week later she's back to school. Meanwhile, I'm strugling to get out of bed, just because my hamster died. I feel so shitty."
"Dude, don't beat yourself up about it. The water that hardens the egg softens the potato. "