A method for disassembling and/or assembling an underwater section of a retractable thruster unit of a swimming vessel. A supporting cradle is detachably fixed to the underwater section from below, and the thruster unit is lifted so that the underwater section is at least partly brought inside the well formed in the bottom of the vessel. Lifting wires of an auxiliary lift are connected to the supporting cradle for supporting the thruster unit. Water is drained from the well, and the lower gear is disengaged and the intermediate section and the lower gear are sealed. After that water is let to ingress into the well and the underwater section is lowered down by the auxiliary lift to dock bottom or sea bed, from where the underwater section is picked up with a crane.