In whatever manner the ship was constructed, they were not going to have to row it to get to their destination. They had to get out into the currents of the sea and into the winds, and the Lord would guide them to the Promised Land. We read that they were driven by the wind, but they still needed the Liahona to work. This suggests their ship may have had sails or rudders or something so that enabled it to be guided.
Thus, they still needed the Liahona. They could not just sit back and allow the wind to blow them to the Promised Land. There was a purpose to have the compass working on their ocean voyage. It says they had to steer the ship, and they needed the Liahona to know where to steer it. It also said that the wind blew them back and they were back-tracking for days. It was a combination of the Lord providing the means and them following the path.