Musannaf Ibn Abi Shayba 3:4194

Abu al-Ahwas narrated to us, from Mughira, who said: I asked Ibrahim about praying on a ship. He said: "If he is able to go out, then let him go out. Otherwise, let him pray standing if he is able. Otherwise, let him pray sitting, and he should face the Qibla whenever it (the ship) turns."