Acting Petty Officer Andy Coulson received the award at the bottom of a Gibraltar swimming pool.
He was in the middle of a two-week exercise on the island, which included stints at the military outdoor swimming pool, when he found himself in the midst of the impromptu ceremony.
The 34-year-old Scot, who is based at Horsea Island, said: 'I was not expecting it.
'I mentioned it in passing several weeks ago that I thought it would be good to receive my medal underwater rather than on the parade ground, but I didn't expect anyone to act on it.
'I'd totally forgotten that I mentioned it, but it's something that will live with me forever – certainly one of the highlights of my 15-year career in the navy.'
As well as a long service medal, the award recognises PO Coulson's good conduct over the years as well.
His boss, Lieutenant Simon Leightley, the officer in charge Southern Diving Unit 2, said: 'Andy's been selected for promotion and will soon embark on his professional qualifying course, so it seemed an appropriate send-off to present his medal "in the office" so to speak. I gave a slightly shorter citation than normal – mostly through bubbles.'
Joining PO Coulson and Lt Leightley underwater for the presentation was Warrant Officer (Diver) Robin Rickard, representing the Southern Diving Group.
He joined the navy at 16 as an Air Engineer Mechanic, but transferred to the diving branch 11 years ago.
WO Rickard said: 'I have already had a fulfilling career but am looking forward to continuing my progression.'
NATIONWIDE TEAMThe navy divers based at Horsea Island are part of a nationwide team.
The Northern and Southern Diving Groups are responsible for all diving, demolition and explosive ordnance (bomb) disposal carried out by the navy within UK waters.
They are on call 24 hours a day, 365 days a year, and can carry out underwater engineering for the surface and sub-surface fleet on a planned or emergency basis.
The two groups also do bomb disposal duties for the police and coastguard, with Portsmouth's Unit 2 working from the Humber estuary to Devon.
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