1983 ODay 28 with a slow leaking thru hull fitting that feeds the toilet. Any in water options? By the looks of the rusted off valve handle I don't think trying to tighten the nut would be a good idea. Absolutely no sailboat experience.
I highlighted the operative words in your query.
An underwater leak is a problem that needs immediate attention on a boat.
Primary function of a boat "Keep the water Out of the Boat".
Rusted valve indicates a failure.
Lack of experience is not an issue, ignoring the issue will become costly. Due to lack of sailboat experience, and by that I mean boat experience in general, I would suggest you make an immediate arrangement to pull your boat out of the water and address the thru hull by replacing the thru hull and the valve.
If you feel you can do it great. If you are concerned that you need expert help, then ask for it and act upon this issue today.