The former paddle steamer has been repaired and redesigned before returning to its home opposite the London Eye in time for the summer season.
Chief operating officer Nigel Wright said: “Refurbishing the Tattershall Castle presented some unique challenges when compared to pubs on dry land and the project was several years in the planning stage before committing to it.
“The final result is nothing short of spectacular and we’re delighted to have played a part in preserving a London icon.”
In a nod to the ship’s history, the original engine room has been restored and made visible. The new food and drink menu sticks to the nautical theme, with seafood and fish and chips served alongside a choice of 30 rums and a range of craft beers.
The Tattershall Castle closed in January and was taken to Hull for repairs, where previously the boat spent 39 years working as a passenger ferry across the Humber.