Here are a set of instructions for building 15mm models of ancient War Galleys from the Jackson Gamers.
Corvus is a set of free wargames rules for fighting ancient sea battles.
Trireme, from Andy Watkins, is a set of free naval wargames rules. These are of the one-page variety, which means that there is much to recommend them. I also like the staggered movement system based on boat speed, which reminds me of the old Car Wars system.
Oceanus Strategoi is a set of free wargames rules for Ancient Naval warfare, designed for 1/1200 figures
Ramming Speed is a set of free wargames rules for righting naval battles with ancient galleys. The rules are designed for 15mm models.