Alternative for sameday weigh in
Making it "illegal" for a fighter to have his total waterweight below a certain percentage of his own bodyweight.
This will prevent fighters from doing a lot of dehydration at all, while with same day weigh in they might still try to dehydrate and use PEDs to get their waterweight back really quickly.
Or stop stupid trainers from harming their fighters if they think a lot of weight can be hydrated back in a single day without harm.
It seems like the safest choice to me.
(It isn't that hard to measure waterweight either)