The cat had got stuck under the HSC Gotlandia II, which operates in Visby on the island of Gotland, and the crew were spurred into action after hearing strange noises coming from the vessel.
"We heard the cat meowing when we were getting on to work, it had probably fallen into the water and was trying to crawl up. He sat there and was distressed," crew member Stellan Stenberg told Aftonbladet.
Colleague Johan Skärkarl donned his wetsuit and jumped into the water to retrieve the animal, before swimming to the shore with the cat nestled on top of his stomach.
"He is an animal lover and said immediately that he wanted to jump in," said Stenberg.
Once ashore the cat didn't make a break for it after escaping with another of its nine lives intact. Stenberg filmed the rescue and uploaded it onto YouTube in the hope of finding the cat's owners.
"It didn't run away but stayed with us and seemed curious," he told the TT news agency.
He added to Aftonbladet; "I myself have wondered who owns the cat and it would be fun to find out," he said.
It's unlikely that the cat would have got swept out to sea as the ferry only operates during the summer.