Leading Seaman Carl Thomas, 28, from Llanelli in Carmarthenshire, dived in the engine room of the ice patrol vessel HMS Endurance, after it flooded while at sea off Chile in December 2008.
The diver submerged three times over about an hour, battling against a torrent of water in his attempt to repair a seawater valve which had opened while unconnected due to incorrect installation of its airlines.
Thomas, based at Horsea Island’s Fleet Diving Unit, received his award from Commander-In-Chief Fleet Admiral Sir Trevor Soar, for his “outstanding courage and fortitude in the most hazardous conditions”.
Despite his efforts, Thomas could not stop the flow. Fortuitously the ship, which had lost all power, drifted over an area shallow enough for its anchors to be dropped.
This provided sufficient stability to prevent capsize or running aground until tugs could arrive to pump out water and take the ship in tow.
The incident was classed as a “near-loss” situation.
After emergency repairs in Chile, Endurance was towed back to the UK. The ship remains in Portsmouth pending a decision by the MoD over whether to repair or replace it.