Diuretics are prohibited, while stool softeners/laxatives are allowed.
I'll quote USADA:
"CATEGORIES OF SUBSTANCES PROHIBITED AT ALL TIMES (IN AND OUT-OF-COMPETITION):
Diuretics and Masking Agents..."
"EXAMPLES OF MEDICATIONS THAT ARE PERMITTED:
Laxative/Stool Softener: Colace, Correctol,
Dulcolax, Ex-Lax, Fibercon, Fleet Enema, Metamucil,
Miralax, sennosides, Senokot, Senokot-S"
[SOURCE: USADA Wallet Card examples of prohibited and permitted substances and methods. PDF here:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiw3PLS_ejaAhWqx4MKHZQXAdIQFggnMAA&url=https://www.usada.org/wp-content/uploads/wallet-card.pdf&usg=AOvVaw3NIGK6DqUcanvmzZImFCx2]
But it hardly matters because in order to cut the large weight that most fighters do, they've emptied out their system prior to weigh ins. Most fighters switch to an all liquid diet the last 48 hours, and some hardly even drink the final 24 hours. If their digestive tract isn't completely empty when they step on the scale they've screwed up (that goes for those cutting a lot, not those hardly cutting or the HWs that don't cut at all).