A submersible apparatus (10) which can be fully submerged in a liquid medium, includes: a sealed hollow body (12) having a longitudinal main axis (A); at least one driving member (14, 36, 52) for moving the apparatus (10) horizontally and/or vertically, which can co-operate with the medium in which the apparatus (10) is submerged and which can move in relation to the hollow body (12) through an opening (22, 44, 56) provided in the wall of the body (12); and elements (18, 40, 64) for actuating the driving member (14, 36, 52), which are disposed inside the hollow body (12). The invention is characterized in that the opening (22, 44, 56) is sealed by a flexible membrane (24, 50, 58) which can deform and co-operate with the driving member (14, 36, 52) as the driving member (14, 36, 52) moves through the opening in order to move the apparatus (10).