Corvus is a set of free wargames rules for fighting ancient sea battles.
David Heading offers a set of Renaissance Naval Rules.
Togo is a set of free wargames rules for naval battles during the 1904 – 1905 Russo-Japanese war.
Schnell Rules is a set of fast play rules for WW2 coastal forces.
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.