Two-year-old Todd's six-hour, 16-km marathon surprised canine experts and delighted his owner, newspapers said on Wednesday.
"To swim that far is incredible, especially in the sea. I'm seriously impressed," Freda Scott-Park of the British Small Animal Vetinerary Association told The Times.
Owner Peter Loizou, who spent four hours searching for Todd after discovering his canine crewmember had jumped ship in the Solent, said it was a miracle he had survived.
"He swam across the waves, across the currents to get home," he said. "I am so pleased to see him, he is like a child to me."
Pet and master were finally reunited after Todd clambered ashore after swimming up the River Beaulieu in Hampshire and police scanned a microchip in the dog's ear.