Romance on the High Seas (1948)

Although Mike (Don DeFore) and Elvira Kent (Janis Paige) are in love, each suspects the other of unfaithfulness. Elvira plans an anniversary cruise to Rio, but, when Mike cancels at the last minute and encourages her to go alone, Elvira is convinced he's seeing someone else. She asks singer Georgia Garrett (Doris Day) to go on the cruise in her place, unaware that Mike has hired a detective, Peter Virgil (Jack Carson), to take the cruise to spy on "Elvira."