"Construction for the Neptune Pool spanned 1924-1936. Three swimming pools were built on this site, each successively larger. Initial plans for the site called for a "Temple Garden" with an ornamental pool and temple structure. On March 31, 1924, W.R. Hearst wrote in a letter to Julia Morgan, "I am sending back the plan of the temple garden with the suggestion that we make the pool longer than it is, as long as a swimming pool. Mrs. Hearst and the children are extremely anxious to have a swimming pool!"

Charles Cassou designed the series of sculptures for this "Neptune Pool"; Julia Morgan was the architect.
I believe this Pantheon-like building is the bath house!
http://www.hearstcastle.com/history/pools.asp