In this paper an underwater laser cutting process for the decommissioning of offshore geotechnical structures after usage is presented. The process equipment consists of standard laser components which are protected against water penetration to achieve a process that is affordable even for medium-sized companies operating in the marine sector. The cutting is carried out on 10 mm thick steels plates of S355J2+N in a laser flame cutting process with oxygen used as cutting gas. In order to achieve process robustness with respect to economic cutting speeds, the influence of the applied cutting gas pressure on obtainable cutting speeds is investigated. The determined parameters are then transferred to cut up to 50 mm thick plate packages. For quality control in future applications the process can be monitored through an optoelectronic monitoring system based on process emission detection.