That allows you fill the underdeck, built-in ballast tanks to lower the boat's center of gravity for a more stable ride. Either that, or it's simply used to fill the onboard fresh water tank.
OK, you really need to start learning your own boat
I can't say for sure on your boat, but it's often on one side or the other, near the aft end of the cockpit, under the gunwale. It might say in your manual (download it, if needed). Either way, once you're on the boat, you can't be more than 22' from finding it.
Looks like a different spot for such a fill. Wondering if it was added at some point.
You can check GW website and see if there is a brochure for the year on the site. Maybe reach out via their cust svc weblink and ask?
One thing that caught my eye was that it appears to be a Marinium deck fill, which would place it as possibly being original to his '85. Although, of course that's not to say that it wasn't added at some point, either.
I learned a couple things about my boat, since I haven’t used the freshwater in 2 years I don’t need it secondly looking at the brochure where the freshwater hose would plug in at , the guy that owned my boat before put bumpers on the inside and covered it up so I couldn’t really use it anyway. So thanks again for the feed back.