Green’s Dictionary of Slang

hot water n.

also warm water

1. [mid-16C+] difficulties, problems; usu. as in hot water.

2. [late 19C] (US) in a restaurant, a cup of tea; also attrib.

3. see hot air n. (1)

In compounds

hot water house (n.)

[mid-19C] (UK tramp) a very low level of lodging house; thus hot-water lodger.