ON THE SHORE OF THE SUNDOWN SEA by T. H. Watkins (Johns Hopkins: $14.95, illustrated) . T. H. Watkins wrote this mixed memoir and polemic in 1973, prompted by the outrage he felt when he saw how overdevelopment had ruined Salt Creek, a small beach about five miles south of Laguna he knew and loved as a boy: "The little terraced bluff overlooking the beach had once harbored a collection of summer tents; now it harbored a collection of summer homes for the rich folks. The little road that leads down to the beach--asphalt now--was chained off. Private Road. No Trespassing. . . . Gone was anything I could fix my memory on, except the sea itself." Watkins recalls the events that shaped his love affair with the California coast: childhood camping trips with his parents; life on a slightly leaky houseboat moored in San Francisco Bay; his futile efforts to save birds caught in a 1970 oil spill. The pleasures of this warm, intimate book are darkened by the awareness that developers and the oil industry have wreaked even greater havoc on the Pacific coast during the intervening 17 years.