I think this was a time-travel fic, but I'm not certain.
What I recall is that after Jusenkyo when Ranma and Genma visit the Amazon Village and in canon they eat Shampoo's prize, Ranma stays away from the table and tries to point out that the food is 1st prize to Genma. Genma ignores him and eats the prize.
Shampoo challenges Ranma as canon, but he somehow gets hot water and reveals Genma's true form. Shampoo winds up fighting Genma and gets defeated.
I can't recall if the hot water comes before or after Shampoo's defeat, it might have come afterwards when Shampoo tires to kill or discipline the panda.
Sorry, my memory is fuzzy, all I recall for sure is Ranma making a point NOT to eat the prize and Shampoo winding up defeated by Genma.