Dominion: Seaside is an expansion for both Dominion and Dominion: Intrigue. As such, it does not contain the material for a complete game.
All you ask for is a tall ship and a star to steer her by. And somebody who knows how to steer ships using stars. You finally got some of those rivers you had wanted, and they led to the sea. These are dangerous, pirate-infested waters, and you cautiously dispatch rat-infested ships across them, to establish lucrative trade at far-off merchant-infested ports. First, you take over some islands, as a foothold. The natives seem friendly enough, crying their peace cries, and giving you spears and poison darts before you are even near enough to accept them properly. When you finally get to those ports you will conquer them, and from there you will look for more rivers.
One day, all the rivers will be yours.
Dominion: Seaside (Second Edition) comes with nine new types of Kingdom cards, which will be released on their own as an update pack for those who own the first edition of this expansion. A few cards will get minor errata, but these will not appear in the update pack.