You will need a degree in hard knocks, mechanical, structure or civil engineering with some classes in hydro an aero dynamics. You will also need to know how to draw with CAD and hand .
Competitive sailing is really the only place to start for a competitive yacht designer.
You only have to look and see how some of the kiwi designers and how they started. Most started as keen sailors just wanting to make there boats go faster like Davidson in his E class boats. You will only find out what fast is by on the water experience. People like Davidson, Ross, Elliott, BakewellWhite, and many others big boat designers learnt in dinghies . If you are into sailing you have a head start.