A sealing arrangement is used when mounting a thruster in a well box attached to a hull of a marine vessel. The well box has an axis, a bottom flange, a top flange, an annular wall therebetween and hoisting pipes fastened parallel with the axis to the bottom flange radially outside the annular wall. Both the well box and a mounting flange have at least one surface acting as a sealing surface between the well box and the thruster. At least one of the surfaces has a groove for a seal for preventing water from entering the well box. The well box has a rotationally symmetric wall with an internal guide surface and the mounting flange of the thruster has an axially extending rotationally symmetric part provided with an outer surface having means for sealing the mounting flange in relation to the well box.