Wetting the sails down helps hold more wind by closing the gaps and making the sails stand flatter.
Now the sails had to be kept wet, for a wet sail holds more of the wind than a dry one. Water had to be hoisted up from the sea to the towering height of Constitution's yards and spilled down the sails. Even in the humidity that prevailed — the humidity that made the labor so exhausting — the rate of evaporation from that area of canvas made it necessary that the work should be hard and continuous.
Age of Fighting Sale, C.S. Forester (Author of the Horatio Hornblower novels)
Some one suggests "wetting the sails." On this point I would say a few words to you. No doubt wetting swells the threads of the canvas, which then offer a closer surface to the action of the wind ; but if the sails are new they do not so much require it, and the jumping about of the men will do more harm than good. Another disadvantage may arise from their not being wetted evenly, which, in the hurry and excitement of the moment, the chances are they will not be ;
The Yacht Sailor. A Treatise on Practical Yachtsmanship, Cruising and Racing
The general practice among English yachtsmen when the America arrived there in 1851, was to cut and sew the sails so they would form themselves into a bag to hold the wind, although the advantage of flatness of surface for plying to windward was well understood, and the practice of wetting, or as they termed it, “skeeting” the canvas, was well understood and often employed on board racing boats and ships to flatten their canvas.
How Sails are Made and Handled: With a Chapter on Racing Kinks
It appears from theory that the wetting the sails of a ship should increase the advantageous action of the wind upon them, not only by closing up the interstices between the threads of which they are com posed, but likewise making the sails stand flatter. The custom of wetting the sails on board small vessels, the only ones where the expedient is at present practicable, seems to confirm the idea of its supposed utility.
The Mechanics' Magazine, Museum, Register, Journal, and Gazette, Volume 56
Flax fibres swell in thickness with moisture and wetting sails is one way of decreasing porosity and hence increasing effective driving force (Gordon, 1977: 143)
Ancient Boats in North-West Europe: The Archaeology of Water Transport to AD