From A Wiki of Ice and Fire
Seaswift is a war galley of the royal fleet at King's Landing. She is a small ship with big sails. Her captain claims that she is faster than any war galley afloat.
A Clash of Kings
The Seaswift is part of the fleet Tyrion Lannister sends to bring Princess Myrcella Baratheon to Braavos. As the fastest ship, it is to carry Myrcella. If they spot any lone ship they are to run it off or destroy it. Should an enemy fleet give battle the Bold Wind will escort the Seaswift to safety while the rest of the fleet, consisting of the King Robert's Hammer, Lionstar and Lady Lyanna, gives battle.
- ↑ 1.0 1.1 A Clash of Kings, Chapter 41, Tyrion IX.