The Ghost in the Invisible Bikini (1966)

Hiram Stokely (Boris Karloff), a recently deceased man, discovers that he has only one day to enact a good deed in order to avoid eternal damnation. Hiram's scheming lawyer, Reginald Ripper (Basil Rathbone), is determined to get his fortune, and is willing to bump off potential heirs such as Chuck Phillips (Tommy Kirk) and Lili Morton (Deborah Walley) to achieve his goal. As Chuck, Lili and others have a lively pool party, Hiram recruits the ghost of his dead lover to help set things straight.