Aquanautica Imperialis is a set of rules for naval games in the world of Warhammer 40K.
Pz8 is a series of two-page wargames rules. Among them are is a set of free wargames rules for coastal actions.
David Heading offers a set of free wargames rules for Renaissance Naval Battles.
Fleet of Battle is a set of free wargames rules for playing out battles between fleets of ancient galleys. Think Ben Hur. There also are several pages of free print-and-play galleys for use in lieu of lead miniatures. The rules require a knowledge of the commercial Field of Battle wargames rules.