Interesting discussion going on here...!
For basic ocean wave formation, please check fluid dynamics theory at http://en.wikipedia.org/wiki/Ocean_surface_wave
Breaking waves appearing in the form of whitecaps over the sea surface is a wind-driven feature that occurs at a wind speed greater than about 4 m/s (> 8 knots or > 3.5 Bft). Below that wind speed, whitecaps are simply not observed.
Whitecap coverage of sea surface is proportional to both wind duration and fetch.
As wind blows over the sea surface greater than about 4 m/s, a "whitecap" of bubbles and streaks is created on the breaking crest of an ocean surface wave. Whitecaps are formed when large numbers of bubbles produced by a wave rise to the surface, and the bright white area of a whitecap arises from multiple light scattering by elements of size comparable to or greater than the wavelength of visible light. These elements are clusters of proximate, closely packed bubbles at or near the surface of the breaking crest of an ocean surface wave (source http://www.springerlink.com/content/j0647331h845710t/
So to answer the initial question, yes, it is possible to start and sustain the planing in the absence of clearly visible whitecaps. Formula can do that as of 6-7 knots of wind speed. However, it is impossible to start and sustain the planing as of 4 knots of wind with any type of windsurfing gear, except when the forward motion is provided by the power of a wave pushing the surfing hull down the line.