The fire-ship's topsails are set for most of the climax, but when the Defiant sinks it, all its sails are furled.
In the last scene, Sir Alec Guinness orders all larboard (left side) guns to bear on the fire ship. In the next shot, the starboard (right side) cannon fire. Ironically, the term "larboard" was changed by the navy into the current term "port" precisely because it was too easy to mistake it for starboard.
During the final battle, the Captain tells the helmsman, "Full astern." That would mean to go into reverse. That action is not possible on a sailing ship.
Newly pressed Landsmen, that is to say men with little to no seaman experience conscripted into the Navy, would never be required to climb the shrouds and set sails during their first days on board. These duties were reserved for sailors known as Topmen who were typically the most experienced crewmen aboard.
During the battle scene, cannon do not recoil when firing.
At one point a sailor is heard singing "Oh Shenandoah." While the origins of this folk song remain unclear, most believe it originated in the early 1800s and thus would not have been known at the time of this movie's 1797 setting.
As the second sailor is buried at sea, you can see a rope tied to the body leading back along the ship---obviously so they can haul the dummy back on board after the scene ends.
Vizard says "We do a full try out tomorrow, at eight bells," but he doesn't say which eight bells (00:00, 04:00, 08:00, 12:00, 16:00, 20:00 or 24:00).
The blanket used to cover Vizard at c.95 minutes suddenly becomes bright blue.