I think Australia is probably unique in that we have a night VMC classification. Following a number of tragic accidents where folks died because they didn't have the instrument skills on absolutely black, black nights they introduced a ruling that for night VMC there must be a discernible horizon. From experience I know that being over water on a pitch black overcast night with no celestial lighting you can't get more IMC.
https://www.casa.gov.au/standard-pag...nible-external