David Heading offers a set of free wargames rules for Renaissance Naval Battles.
Rules: Naval
War Galley
War Galley is a set of free wargames rules for ancient naval warfare. It very cleverly uses matchboxes and matchsticks for play.
Trireme Game Rules
The Trireme Battles blog offers a set of free wargames rules for trireme battles.
Naumachiae Ancients Naval Rules
Naumachiae is a set of rules for ancients naval warfare for 1/12000 miniatures.
La Felissima Armada Naval Rules
La Felissima Armada is a set of free wargames rules for naval skirmishes between England and Spain in the Elizabethan Age.