Pursuit To Algiers is a 1945 Sherlock Holmes movie starring Basil Rathbone and Nigel Bruce as Holmes and Doctor Watson respectively.
Holmes and Watson are approached to escort a king back to his Mediterranean homeland.
The movie is set mostly on the boat, as enemies of the king try to kill him.
They arrive safely though in the end and the real king was the steward.
The one they were escorting was a decoy. Holmes knew but not Watson.