Ahmad ibn Muhammad from Ibn Abi ‘Umayr from more than one of our companions from Abu Abdillah عليه السلام, peace be upon him, who said:
Imam al-Sadiq عليه السلام said: “Prayer on a ship is by gesturing (ima’).”
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 3, p. 330