From the back of the box:
Every land in Cerilia depends on the sea for military strength and trade. Many times, the quality of a regent's fleet and the naval power at his command prove the deciding factors in his success or failure as a ruler.
This supplement expands the Birthright domain and War Card rules to handle sea travel and combat. Players can use the sample navies included for Anuirean, Khinasi, and Rjurik kingdoms as guidelines to tailor fleets to meet the needs of their own domains. Special maps illustrating the maritime areas surrounding Cerilia make it easy to add a naval dimension to any campaign.
The Birthright Naval Battle Rules contain:
The Seas of Cerilia, a 16-page book detailing the rules for naval battles in the Birthright game, including sea movement, ship types, naval armament, sea trade routes, and battles on the water.
A set of 51 naval War Cards featuring game statistics for the vessels of all the lands of Cerilia, as well as special wind cards governing movement at sea, and cards describing naval war magic.
Counters for indicating crew capacity and ship movement rate.
A full-color, poster-sized naval battle mat for ship-to-ship combat.
These naval battle rules first appeared in the Cities of the Sun campaign expansion. If you already own that product, it is not necessary to purchase this one.