Cooking is forbidden on Shabbos but re-heating is permitted provided a certain number of conditions are met, e.g., the food is solid and already cooked (at least to a large extent), the device used to re-heat cannot be adjusted up and down and one doesn't give the appearance to cook.
In your case, assuming the hot water was heated before Shabbat (as it can't be heated on Shabbat itself), it would be permitted to place pre-cooked food or a pot with pre-cooked stew in a large pot of hot water (since no one normally cooks this way, a.k.a. a bain-marie).
Note this is valid is gravy is mixed into the rest of the food (e.g., if the food is moist). Reheating water of liquids on their own is always forbidden.
For sources, see e.g., here for general laws of cooking, here for re-heating, and the bottom of here for placing fully cooked food on a heat source that is not normally used for cooking.
I checked this with a Rosh Kollel in Bnei Brak but, of course, consult your rabbi
before implementing anything you learn here.