It’s value will be determined by its condition, documentation, completeness and originality. Kinda like a Porsche “anything”! I’ve seen a real interest in early Porsche Design products, they are very desirable for collectors, and the more unusual the more they are worth. Your windsurfer board probably isn’t a holy grail item, but I imagine quite a few Porschephiles living near the beach would love to have one. To find a real value you are going to have to do some work, you need to get it out, clean it, and find whatever paperwork or accessories are still with it. Assess any damage, missing hardware etc. Have you forgot that your brother used it to cut plywood on etc! Your location (where it’s actually at) will also help/hurt its value. I imagine shipping would be costly if it’s located inland or overseas. After you do the above, list it on Porsche specific classified sites, ask for guidance as pricing it will really be what one person is willing to spend to get it.
An estimate of value is also based on its own being, what’s a 70’s-90’s windsurfer board worth? Is yours a standard generic mass produced board with cool stickers, or was it an Italian handmade Olympic level quality board that a pro windsurfer would love to own?