Falconer’s Marine Dictionary (1780) has this definition of a water-sail: “a small sail spread occasionally under a lower studding-sail or driver boom, in fair wind, and smooth sea.” Studding sails and drivers both have a boom holding the sail’s foot.
Here’s the dictionary’s definition of a driver: “an oblong sail, occasionally hoisted to the mizzen peak, when the wind is very fair. The lower corners of it are extended by a boom or pole, which is thrust out across the ship, and projects over the lee quarter.”
—Ed.