White caps form at 10 knots. Open water, semi-protected water, protected water - the caps still form at 10 knots, it's the size that varies. Open ocean water, the waves/swells are about 1 meter height with a 5 meter wave length. In protected water, the waves/swells could be 3 cm high with a 10 cm wave length.
This may or may not be the actual proportions, just my recollection from numerous observations and some very basic knowledge of waves. I am no expert.
The cap is a small piece of white water breaking off the top of the wave. So when you look at protected water in 10+ knots of wind, and don't see white caps, look closer because they are probably there.
Also, in protected water, the wind speed 1-2 meters off the surface may be higher than what it is at the surface, so a sail could have 10 knots of wind in it while the wind speed at surface is less than 10 knots. No caps to be seen, but the board has enough wind to plane.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.