Schnell Rules is a set of fast play rules for WW2 coastal forces.
Mike Callan offers this set of free wargames rules for American Civil War Ironclads. The set originally was published in Wargames Illustrated 14.
Trireme, from Andy Watkins, is a set of free naval wargames rules. These are of the one-page variety, which means that there is much to recommend them. I also like the staggered movement system based on boat speed, which reminds me of the old Car Wars system.
Tough Men With Iron Balls is a set of free wargames rules for fighting 7 Years War and Napoleonic Naval Battles.
Heart of Oak, Skin of Iron is a set of free wargame rules for naval combat in the years 1852 – 1870.