Barb and I have been looking for this ad for years and finally found it tonight on Google Books website. In early 1966, she, Margo Maggiani and Mary Lou Hollywood were chosen to model for a MerCruiser ad photo shoot at Lake Nacimiento.
The ad was published in Popular Mechanics March 1966 special issue on boating. Barb is driving the boat, Margo is sitting by her and Mary Lou is in the back seat.
They didn't get paid in cash, but could choose color-coordinated outfits from Paso Robles Mercantile.
One exciting tidbit: Barb almost wrecked the boat after they told her to drive fast and close to the huge boat they were shooting from. She came a little too close and almost hit it. Barb's "Please don't tell the owner that I almost wrecked his boat" was answered by, "I don't have to: he's standing right here!"