Benefitting from Melacha

Question: One of our teenage children added water to our Shabbos kettle on Shabbos mistakenly thinking that it was muttar. We need hot water to make baby bottles. Can we use the water?

Answer: The Gemara (Shabbos 38a; Bava Kamma 71a; Kesubos 34a; Chullin 15a) teaches that chazal enacted a decree that one cannot benefit from a forbidden melacha, (maaseh Shabbos) that was performed on Shabbos. There is a machlokes as to whether this only applies to one who purposely transgresses a melacha and whether it applies to everyone or just the one who performed the melacha.
Rambam (Shabbos 6:23) and the Shulchan Aruch (OC 318:1) follow R’ Yehuda who maintains that one can never personally benefit from melacha that one did on purpose (bemeizid) though others may benefit from it after Shabbos. If one did so accidentally (beshogeg), however, then all may benefit from it after Shabbos.
Tosafos (Chullin 15a) and Ritva (Shabbos 38a) however, follow R’ Meir who allows one to immediately benefit from a melacha performed beshogeg. The Mishna Berura (318:7) writes that the Vilna Gaon (OC 318) follows this view and one may rely on this view in time of need.The Mishna Berura (318:6) defines one who performs a melacha due to not knowing the halacha as shogeg. Thus, one who accidentally cooked food on Shabbos would usually have to leave it until after Shabbos before consuming it unless there was a particular necessity, though that is difficult to define.In conclusion, one would be allowed to use the water for baby bottles as that constitutes a real necessity. One shouldn’t use this water to make oneself a hot drink, however.