According to Sir Christopher Lee, Hammer had built a full-sized galleon in some sand pits on a steel structure under the water. Although warned not to have too many people on-board at once, one day the tea boat was lifted onto a platform level with the water with too many people getting their tea. The ship capsized throwing most of the cast and crew in the water. Lee was on the poop deck and luckily managed to hold onto the rail. Thankfully no one was drowned or seriously hurt.
According to Sir Christopher Lee (in "The Films of Christopher Lee") in the final sword fight, "I was badly bashed about in that. I had a pretty tough fight on the deck of a ship and was bleeding all over the place."
Although Andrew Keir (Tom) was less than four years older than John Cairney (Harry), he played his father in the film.