Mark Thorne dropped his ring as he helped launch the inshore lifeboat in Weymouth Harbour in Dorset in 2009.
Despite attempts to find it on his return, it was lost until his friend Steve Woolford spotted it during a walk at low tide yesterday.
Thorne said he was "really, really chuffed".
When he returned from the emergency in 2009, he searched under water and used a metal detector in an attempt to find the ring.
He said he had "given up hope" of finding it, despite occasionally returning to look for it.
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He said he remembered the search for the ring and thought "you never know, it's worth a look", before he spotted it glinting in the mud.
Thorne, who was married in 2004, was able to identify the platinum ring from its size and a distinctive dent.
"I think I owe Steve a bottle of whisky," he said.