OK, I know "pre-modern" is broad, but as a medievalist with some classical background, I can't think of any early descriptions of seasickness. Do any examples spring to mind?
Do you want prolonged descriptions or just references? Huizinga cites two occurrences of it in sources:
In the Debat des Herauts d'Armes de France et d'Angleterre, an anonymous 15C text that is more or less what it says on the tin, the French author explains the French nobility prefer to fight on land, since on the sea (translated in the Autumn edition) "there is danger and loss of life and God knows how awful it is when a storm rages and seasickness prevails which many people have to bear."
Apparently in England, there was a designated official whose job responsibilities either included or were entirely to hold his head up while he puked crossing the English Channel. Huizinga notes that in 1442 this office was fulfilled by a man named John Baker; it was subsequently inherited by his daughters. There is not a named source for this, but you might be able to reconstruct which one through the bibliography or with the date in mind.