A one-of-a-kind Lake George waterfront compound with a spectacular mansion, guest cottage and 33 income-generating boat slips is being offered for sale. Known as the ''Green Harbour Mansion,'' the 3.3-acre property occupies the tip of Cooper Point on Lake George (1,585 feet of lakefront) and was built in 1913 for Frederick F. Peabody. The property is anchored by the 14-bedroom, seven-bathroom, three-story mansion. The basement space of the mansion has been converted into two separate two-bedroom, one-bathroom seasonal rental units. There is also a four-bedroom, two-bathroom waterfront cottage. The property's landscaped grounds include a rock waterfall, a rope swing and a large gathering area on the point that provides panoramic views of Lake George.