Very relevant thread Alby given windsurfing seems to be an old fart's sport these days.
i previously got a pair of single vision prescription sunnies via an online shop. They were $40 Cancer Council frames and worked great, until I lost them (on shore) at Fangies the other day (there's something about Fangies, I've also managed to leave my thongs and downhaul puller behind there

). So I recently got a pair of generic Specsaver branded plastic frames via the local shop (no doubt OPSM etc also do similar). What I was very surprised with was their quote for multi vision ("graded") lenses. Usually that's a few hundred dollars out of pocket,but with the SS deal plus private healthcare rebate, it was less than $100 cash. Very happy with that price and they should be good for windsurfing