Abstract: Oxygen generation by means of electrolysis is used for underwater swimming. The apparatus includes a back-pack containing an oxygen generator, a battery, a storage tank and a purifier, plus breathing equipment, including hoses and mask. The generator comprises a cylindrical housing in which an electrolytic cell is rotatably mounted in such a manner that its center of gravity always maintains the electrode of the electrolytic cell in a vertical attitude irrespective of the pitch or diving position of the swimmer in the water.