A dog believed to have drowned after disappearing from a fishing boat off the coast of California five weeks ago has been found on an island owned by the US Navy.

Luna, a blue-eyed German shepherd-husky, apparently swam 3.2 kilometres to San Clemente Island, off the San Diego coast, where Navy employees found her on Tuesday, Naval Base Coronado spokeswoman Sandy DeMunnik said.

"It was a long haul for a puppy across treacherous terrain," Mr DeMunnik said.

"She was a little thinner, but none the worse for wear."

Luna, who is about 18 months old, went missing before dawn on February 10 as her owner Nick Haworth and a crew member hauled in a catch, Mr DeMunnik said.

Mr Haworth searched the waters for two days backed by Navy personnel, who also searched the island for a week, he said.

The San Diego fisherman presumed Luna was dead 10 days after she went missing, posting photos of the dog on Facebook and writing "RIP Luna".

Luna likely spent no more than a day in the water and was found sitting on a road on the island.

"Beyond stoked to have Luna back," Mr Haworth wrote on Facebook. "I always knew she was a warrior."

Luna's owner was due to be reunited with his beloved dog on Thursday evening (local time), the Navy said.

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Reuters