If there's the potential to get a bike frame down to 300 grams one can only imagine the benefits for windsurfing.

I was just watching that.
I immediately wondered if it was applicable to masts - not just masts of course.
It sounds like it should be.
Masts should be lighter because the epoxy graphene matrix is a lot lighter, stronger AND tougher.
It's a way off though according to that vid.

great for fins, might be able to go ploughing with less damage

Head already make squash racquets with Grapheme. They are readily available and, supposedly, incredibly strong and lightweight. I have a 'heavy' one and it weighs 135grams.