Strap Kelp (Lessoniopsis littoralis)
A distinctive perennial kelp, growing in the low intertidal zone on the rocky outer coast. Note the large, compact holdfast, the thick and gnarled trunk-like stipe (or stem), and the abundant strap-like blades (with visible midribs).
Mature plants may have up to 500 blades. The blades can reach up to 1 meter in length (see below).
Strap Kelp grows in sites "exposed to the full force of surf" (Marine Algae of California by Abbott and Hollenberg). Or, according to Dawson in Seashore Plants of California, it is "very well adapted to withstand the most violent agitation of the sea."
If you see Strap Kelp, it's a good idea to be very cautious as the ocean is bound to be dangerous (to inflexible people without strong holdfasts) wherever it grows.
Here's one more photo (below) with both Strap Kelp and Sea Cabbage (Saccharina (= Hedophyllum) sessile) — the latter with shorter, broader blades.
Most of these images were taken at Van Damme State Park. The second photo is from Bodega Head.