A sealing structure for an underwater pump comprises an elbow pipe having a first flange formed at an inlet thereof with an inclined inner surface; an underwater pump having a second flange formed at an outlet thereof; a guide member attached to the second flange and disposed between guide bars vertically mounted under the water so that the guide member is vertically movable along the guide bars, wherein the guide member comprises an annular hollow portion disposed between the outlet and the inlet; and a gasket mounted to the annular hollow portion, wherein the gasket comprises a tapered front portion with an inclined surface that makes close contact with the inclined inner surface of the first flange.