Away with Honour is a set of free wargames rules for naval warfare in the Middle Ages by Tony Cullen.
Rules: Naval
Der Tag Naval Rules
Der Tag is a set of free wargames rules for WWI naval warfare in 1/6000 scale.
Ram or Die – Battle of Salamis Rules
Ram or Die is a set of free wargames rules (and paper figures!) for refighting the Battle of Salamis.
Seas of Lead WWII Naval Rules
Seas of Lead is a set of free wargames rules for World War II Naval games. Along with the rules are some good looking unit data cards that they made using the Magic Set Editor.
WWII Naval Rules
Here’s a set of very simple WWII Naval Rules.