The animal slipped 10ft (3m) and swam for about five minutes before it was rescued by crew onboard a boat that was leaving the docks.
Deckhand Joe Missen named the fox Splash “because that is what he made”, he said.
Although the fox was “cold” after its ordeal, it was otherwise in good health when it was returned to shore, he said.
Mr Missen, who was onboard The Beaufort, said: “They [foxes] go across the lock gates but he slipped and fell into the sea.”
He added: “He was cold and when he got back on dry land he ran off and looked back as if to say, ‘thanks’.
“It was lovely to see him running off because it would have been an awful death for him.”
Coxswain Steve Batcup described the animal as “the luckiest fox in the world”.
“If we had not been there he would not have been able to get back out of the water,” he added.