The HSC Gotlandia II was docked on the island of Gotland when the crew noticed strange noises coming from below the vessel, TheLocal.se reported Sunday.
"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.
Johan Skarkarl put on a wetsuit and jumped into the water to rescue the cat. He swam back to shore with the cat sitting on top of his stomach, the website said.
"He is an animal lover and said immediately that he wanted to jump in," Stenberg said.
Stenberg said the cat didn't immediately run away when they reached the shore, so they took a video of it to try to find its owners.
"I myself have wondered who owns the cat and it would be fun to find out," Stenberg said.