The unnamed woman was taken to hospital in the town of Pula on Sunday.
She fell from the Norwegian Star cruise ship, about 95km (60m) off Croatia’s coast shortly before midnight Saturday.
A Croatian rescue ship was scrambled and found her swimming Sunday morning not far from where she was believed to have fallen.
The ship’s captain, Lovro Oreskovic, said that she was exhausted.
He said that “we were extremely happy for saving a human life”.
A Norwegian Cruise Line spokesperson said: “A report was made that a guest aboard Norwegian Star had gone overboard while the ship was returning to Venice.
“As soon as the report was made the ship began a search and notified the appropriate authorities.
“The ship will be delayed on her return to Venice.”
The circumstances of the incident are being investigated and the British Embassy in Croatia has been informed.
The cruise line’s website says the Norwegian Star was “recently refurbished” and features “15 delicious dining options, 10 bars and lounges, a sprawling spa” and an “always exciting casino”.
The site adds that the cruise offers “tonnes of fun for kids of every age”.