As an idiomatic expression, "Hot water" = "trouble".
Are there idiomatic expressions for warm/cold/dirty water, which mean something other than a literal adjective describing the temperature or condition of water?
> 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.
> 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.