When collecting wave measurements with a Signature or AWAC, waves from 8-10 m/s wind in sheltered areas seems to pose no problem at all. Peak storms (all the way to hurricanes) show the same thing - we have seen great AST data all the way up to 8-9 m significant wave height.
White-capping occurs when a wave becomes too steep to maintain a smooth surface, and breaks at the top. The build-up of a large storm with breaking waves that entrain large clouds of bubbles causes a clear increase in the number AST drop outs. Acoustically this makes sense since scattering from bubble cloud can give very high signal returns, although this is not the case in every white-capping scenario.
The backup solution when white-capping poses a problem is to measure velocity below the surface or pressure. Since the problem only seems to occur when there are large events (Hs > 3-4 m) the orbital velocities are strong and the pressure fluctuations significant so the backup solutions works quite well.