What? It's not a phrase at all. Well, you collected some interesting random answers anyway.

used for talking about a particular type of situation

young couples thrown headfirst into the unknown waters of parenthood

stormy/turbulent etc. waters (=a difficult situation): There seemed to be stormy waters ahead for the administration.

murky waters (=a mysterious or frightening situation): Someone mentions drugs and the waters turn very murky.

https://www.macmillandictionary.com/dictionary/ame...

https://www.mswmag.com/editorial/2016/06/winning_t...

He who wins a situation which is very complex or difficult. ...the man brought in to guide him through troubled waters.

https://www.collinsdictionary.com/dictionary/engli...

https://en.oxforddictionaries.com/definition/us/tr...

that rivers are fast