A successful energy transition requires grid-scale electric energy storage and Carnot batteries represent a potential solution. BattMarines is a Carnot battery concept relying on storing large quantities of high-pressure steam ready to be expanded in a conventional steam power plant. The key idea is to take advantage of high pressure ambient at large sea depths, so that a thin shell vessel can be utilized to store high quality steam at low cost. Power plant is built on a platform/ship with steam pipeline connection to the vessel on the sea floor. The thesis considers the general concept including a rough description of plant configuration, thermodynamic efficiency, insulation requirements and potential materials.