The invention relates to an underwater cleaner (1), especially for a swimming pool, comprising a housing (2) in which an especially battery-powered pump having an electric motor (4) and an impeller (3) is arranged, wherein the housing (2) comprises an inlet opening (23) and an outlet opening (22) for a flow path, said flow path forming a first channel section (13) originating from the first inlet opening (23) and a second channel section (14) accommodating the impeller (3), wherein the second channel section (14) is arranged in an inclined manner in relation to the first channel section (13), and wherein the impeller axis (18) is arranged in an inclined manner in relation to a normal (24) to the opening cross-section of the inlet opening (23). The ratio A/B of a first cross-sectional surface A to a second cross-sectional surface B is less than 2.8, wherein the first cross-sectional surface A is defined by the smallest flow cross-section in the first channel section (13) and the second cross-sectional surface B by the smallest flow cross-section of the second channel section (14) directly at the pump inlet.In order to obtain the simplest possible underwater cleaner (1) with the lowest production costs, which offers high suction power with low flow losses even in the case of impellers (3) of relatively large diameters, the first channel section (13) converges continuously into the second channel section (14).