> hot water - n. a difficult or dangerous situation

https://www.merriam-webster.com/dictionary/hot%20water

> warm water - n. an ocean or sea not in the arctic or antarctic regions

https://www.merriam-webster.com/dictionary/warm%20water

> cold water - n. depreciation of something as being ill-advised, unwarranted, or worthless. e.g. threw cold water on our hopes

https://www.merriam-webster.com/dictionary/cold%20water

Seems that what makes sense to be in dictionaries is already there.