From the publisher's website:

All you ask is a tall ship and a star to steer her by. And someone who knows how to steer ships using stars. You finally got some of those rivers you'd wanted, and they led to the sea. These are dangerous, pirate-infested waters, and you cautiously send rat-infested ships across them, to establish lucrative trade at far-off merchant-infested ports. First, you will 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 close enough to accept them properly. When you finally reach those ports you will conquer them, and from there you will look for more rivers. One day, all the rivers will be yours. This is the 2nd addition to Dominion. It adds 26 new Kingdom cards to Dominion. Its central theme is your next turn; there are cards that do something this turn and next, cards that set up your next turn, and other ways to step outside of the bounds of a normal turn. Dominion: Seaside is an expansion to both Dominion and Dominion: Intrigue. As such, it does not contain material for a complete game. Specifically, it does not include the Treasure, Victory, Curse, or Trash cards. Thus, you will need either the base game or Intrigue to play with this expansion and have experience playing Dominion with either of the first two games. It is designed to work with either or both of these sets, and any future expansions that may be published. We hope you enjoy this expanding world of Dominion!