Abstract: An apparatus for cleaning the underwater portions of a boat having at least one cleaning unit with an ejection element for generating a jet of cleaning fluid under high pressure, and a manifold for generating jets of gas under low pressure. The manifold enables a space to be defined around the cleaning fluid jet in which space density is considerably reduced relative to the density of the surrounding water, such as a space that is free or practically free from water; this space extends from the ejection element to the surface for cleaning of the underwater portions. The apparatus is applicable to cleaning boats, such as sail boats, motor boats, and canal boats, without it being necessary to take them out of the water.