An apparatus for preventing seawater from penetrating into a compressor module during lowering to or retrieval from a compression plant on the seabed, with optional discharge of residual production fluids, as for instance hydrocarbons, from the compressor module prior to retrieval, wherein the compressor module comprises an electric motor (1) and a compressor (2) which are respectively connected via at least one shaft (8) and are arranged in a common pressure shell (3), an inlet pipe (5) and an outlet pipe (6) to and from the compressor module which respectively are provided with an isolation valve (7, 7′). The compressor module is distinguished in that it includes at least one filling pipe (16) which has a shut-off valve (15), at least one drainage pipe (12) which has a shut-off valve (13), and which is located at a lower end of the compressor module, and at least one overflow pipe (14) which has a shut-off valve (17), and which is spaced apart from the at least one filling pipe (16), and that prior to lowering to or retrieval from the seabed, with optional discharge of residual production fluids prior to retrieval, the compressor module is filled with filling fluid via the at least one filling pipe (16) until overflow of filling fluid through the at least one overflow pipe (14). The present invention also relates to a method which correspondingly prevents such penetration of seawater into and discharge of residual production fluid from the compressor module.