Q. After meals on Shabbos, is it okay to lightly soak soiled dishes with cold water?

A. It is forbidden to prepare on Shabbos for the weekdays even on arrangements that do not qualify as melochos by themselves, and involve only slight effort such as soaking dishes or fetching wine (Mishna Berura 503: 1).

However when there is a likely possibility that they will be still needed during Shabbos, even after Seudo Shlishis (ibid. 323: 28), or they create a foul odor or will attract insects, they could be soaked in cold water as needed.(Sh’miras Shabbos Kehilchoso 12: 2)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a