The brother of Jared has three concerns: internal light, seeing the night sky, and fresh air.
The first set of Jaredite barges (presumably for traversing the Mediterranean) evidently did not have watertight “lids” on their vessels. If they had, the Jaredites would have already dealt with these difficulties. Watertight tops appear to be a modification of the previous barges’ design, no doubt triggered by ocean-going requirements.
The Jaredites would not have been concerned about darkness at night, so the desire for light was related to daytime conditions inside the barges, where the lid shut out daylight. The watertight/airtight construction of the barges also meant a ventilation problem, again an artifact of this new construction.
The punctuation in the “steering” sentence makes it modify the question about light: “there is no light; whither shall we steer?” But the questions have to be separate issues. Navigating on the open ocean requires access to stars, the opposite problem from the access to daylight. But both are caused by the same circumstance: the “lid” covering the top of the vessel.