RE: Windsurfing after a back injury
Core muscle strength is key, but with a disc issue, you had better connect with a sports medicine physician for guidance.
Some core strengthing exercises may be counter productive with a bulging disc. Others may be great. You need someone to help you discover what you need to do to sail safely.
I have had back problems too, but not disc related. Since I started a routine of exercises focusing on the core as well as the whole body, things have been 100% better. I generally work out 3 x week but I usually count a sailing session as a work out.