A subsea drying system (10) for drying a chamber (12) underwater has an upstream piping (24, 26, 28) for conveying a pressurized drying fluid from a source (18) to the chamber (12); and downstream piping (36, 38, 40) for expelling liquid displaced from the chamber (12) by the drying fluid in an open-loop dewatering mode. In a closed-loop dehumidifying mode, a recirculation path (54) between the downstream piping (36, 38, 40) and the upstream piping (24, 26, 28) conveys drying fluid exiting the chamber (12) back into the chamber (12) after passing through a dehumidifier (58). A valve system selectively closes and opens the re-circulation path to expel displaced liquid when that path is closed and to recirculate drying fluid from the chamber (12) through the dehumidifier (58) to the upstream piping (24, 26, 28) when that path is open.