I have to agree with Ellen here.
If you wish to become a fully certified RYA Level 5 Windsurfing Instructor, you are going to have to spend some money and put in at least 100 hours teaching level 2 or higher classes.
But, at the end of the day, you will have ALL the necessary skills to teach windsurfing professionally (as a career) anywhere in the world, and RYA certification seems to be what all the larger windsurfing "destination resorts" would like for you to have BEFORE they will hire you.
As Ellen suggests, just becoming a "non-professional" windsurfing instructor in your country or region of residence only requires that you take the courses and gain the experience that your ISAF "governing body" requires for windsurfing instructors in your country.
If you wish to make windsurfing instruction a career, then perhaps you can look at taking the full range of RYA requirements as "going to college" just like you would for any other career.
And, as Ellen suggests, learning to teach windsurfing is one of the most dynamic and satisfying things a person can do. There's always something new to learn, a better way to teach something, etc. with this sport.
Hope this helps,