I wonder why this hasn’t been done before. Eight Bells is a set of paper rules, combined with an iPhone app for playing Napoleonic naval games.
Corvus on Land is a set of free wargames rules to accompany the Richard Lee’s CORVUS (or any other Ancient Naval rules) when played in 1/1200 or smaller scales.
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.