For me it was just the initial water weight I lost in the first 20 hrs or so. So my advice would be to weigh yourself self before starting, then the next day and expect that the difference there is going to come back.
This varies a lot, person to person, because the amount of “water weight” you carry is based around you normal diet. Highly dependent on usual salt and carb intake. The main thing to be shed in the first day or so is liver glycogen stores and this is also what your body focuses on restoring first during recomp.