Don't Surf Stiff-Legged
It is better to surf crouched in a ball than it is to surf stiff-legged.
You can make small, micro-adjustments when your center of gravity is low. Have you ever watched really good finless surfers? They hardly ever stand straight up, they stay low where they can have maximum control over their board—even without the help of fins to keep their board stuck in the face of the wave.
Learn from Nathan Adams and get comfortable surfing from a crouched position. Strengthen those knees, it will be well worth it.
Film/Edit: Thomas Green
Surfer: Nathan Adams Surfboard:
9'9 Sano Special by Almond Surfboards