Aquanautica Imperialis is a set of rules for naval games in the world of Warhammer 40K.
Rules: Naval
Pz8 Coastal Rules
Pz8 is a series of two-page wargames rules. Among them are is a set of free wargames rules for coastal actions.
Renaissance Naval Rules
David Heading offers a set of free wargames rules for Renaissance Naval Battles.
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.