وَتَجُوزُ الصَّلَاةُ فِي السَّفِينَةِ قَاعِدًا عَلَى كُلِّ حَالٍ عِنْدَ أَبِي حَنِيفَةَ، وَعِنْدَهُمَا: لَا تَجُوزُ إِلَّا بِعُذْرٍ.
Translation: According to Abu Hanifah, prayer may be performed sitting aboard a ship even without a further excuse; according to his two disciples, sitting without an excuse is not permitted.
Abu Hanifah treats the special instability of a ship as significant; Abu Yusuf and Muhammad require standing if one is able and has no valid excuse.