Taken from the California Waterways and regulations web.
7009. The Ski Flag.
(a) A red or orange flag measuring no less than 12 inches on each side, in
the shape of a square or rectangle, mounted or displayed in such a manner
as to be visible from every direction shall be known as a ski flag.
(b) The use of this flag will not be construed as conferring any rights or
privileges on its users, and its display will not be construed in itself as
restricting the use of the water in the vicinity of the vessel displaying the
(c) Operators of vessels will, however, exercise precaution commensurate
with conditions indicated.
(d) The ski flag shall be displayed when one or more of the following
(1) A downed skier.
(2) A skier in the water preparing to ski.
(3) A ski line extended from the vessel.
(4) A ski in the water in the vicinity of the vessel. The ski flag shall not
be displayed at any other time.
NOTE: Authority cited: Sections 652, 658, 658.7 and 659, Harbors and Navigation Code.
Reference: Sections 650, 655.3, 658.7 and 659, Harbors and Navigation Code.