[jclark003]

Assuming you are talking about the y valve and not the thru-hull sea cock? If so You can try to replace the handle on the y valve and if unsuccessful, go to a marina and have the tank pumped out a couple times with fresh water flushing. Then get a bucket under the valve as residual waste will leak out when you pull the hoses. Probably should wear gloves, mask and eye protection. Good time to replace any stinky/permeated hoses too.