Described as a "Yachtsman's Paradise on Nantucket Sound," the Osterville estate of billionaire art collector William "Bill" I. Koch is on the market for $15 million. The Cape Cod mansion, called Homeport, was built in 1929.

The seaside residence is 8,653 square feet with nine bedrooms, 12 baths, and surrounded by a gunite pool, guest quarters, and a private sandy beach. There is a 106-foot deep water dock, suitable for an avid sailor such as Koch who is known for his 1992 win of the America's Cup.

He has also made a name for himself as an art collector with a museum-quality assemblage of artworks that have been alternately displayed in both his Cape Cod and Palm Beach residences.

Among the signature artworks collected by Koch are Fitz Henry Lane's luminous The Golden Rule, Picasso's Night Club Singer, and one of Monet's Water Lilies series, which hung on the collector's enclosed sun porch.

Among the home's special features is a coffered-ceiling ship room that Koch had fashioned after the captain's quarters of various ships. He filled it with important maritime antiques. A state-of-the-art wine cellar is adjacent.

Koch reportedly purchased art collector Bunny Mellon's home nearby for $19.5 million.

The Koch residence is listed by Robert Paul Properties / Luxury Portfolio International.