To stop watery diarrhea switch to a diet of either bananas, rice, applesauce or toast or eat all of those foods.
Eating the BRAT diet can help stop diarrhea and then after you stop having the diarrhea you can switch back to your normal diet of other foods.
You should also be sure to stay hydrated with plenty of water, apple juice etc because diarrhea and especially water diarrhea because the diarrhea can dehydrate you.
If the diarrhea continues you should also take some Imodium to relieve and stop watery diarrhea as well as just plain old diarrhea.
The diarrhea should stop within a few days to a week at most and if not then you should see a doctor about it.
Diapers might also be a good idea if you're willing to wear diapers just in case as the diapers will contain the diarrhea and not allow it to go down your pants.