The Lord tells the brother of Jared to “go to work.” That might be an instruction to all of God’s people in all times. There are things we are commanded to do, and perhaps times when we have symbolically remained in place for four years.
God tells the brother of Jared to build barges “after the manner of barges which ye have hitherto built.” That confirms that this is the second time that the people of Jared would cross the waters in vessels. They build them again. Perhaps the instructions we see here were given previously, but even if they were, Moroni elects to include them prior to the major sea voyage. The barges were to be built water-tight and be capable of being covered by water and not sinking. This should not be assumed to mean that they were intentionally submarine-like vessels, but only that they would be able to survive major storms at sea.