Are you sure your toilet uses sea water...that it's not one that's designed to use onboard pressurized fresh water--in which case, the inlet line wouldn't be connected to a thru-hull, it would be connected to a cold water line in the head?
One big clue: Is it a manual toilet that you pump by hand, or is it an electric toilet? The boat's manual will show the standard toilet the builder installs, but it's highly likely that a previous owner replaced it sometime in the last 10 years.
If it's a manual toilet (you pump it by hand), it definitely uses sea water, but if electric it could be either version. Did you get the owners manuals or any other literature for the equipment on your boat that would tell you. If not, do you have any idea of its make/model/approx age? If you don't know, post a photo of it.