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.
Ironclad Draughts is a set of one-page rules for playing out battles between ironclads.
Hunters on the Shoreline is a set of free wargames rules for playing WWII coastal naval actions using 1/600 miniatures. It’s available from PT Dockyards.
The Jackson Gamers have a page with everything you need to know to start gaming Civil War Ironclads