Is the fast of one who drank forgetfully in Ramadan, thinking that the dawn (Fajr) had not yet broken, while admitting that he did not verify the time due to a desire to drink, valid and only requires making up the day, or does it also necessitate expiation?

If you are uncertain whether you drank water before or after Fajr, and it is not clear to you that it was after Fajr, then your fast is valid, and you are not required to make up for it. This is because the default assumption is that the night still persists.