I presume every boater is ignorant of the Colregs and isn't posting a proper lookout. Fortunately, that isn't usually the case, but one can't count on it.
This Saturday, a small catamaran (traveling between two and three times my six-knot speed) was tacking westward in Carquinez Strait, using only 25% of the strait's width. While heading in the same direction, he tacked forward of my course a couple times, getting closer each time. While I'm heading on a steady course, his last tack avoided me.