Since there are no writings or historical records, we have no factual knowledge as to what they knew about the earth or the seas.

It is highly unlikely that they knew about gravity beyond things falling if tossed up, so it would be a very reasonable conclusion for them to believe the earth was flat.

At the same time, IMHO, surely they would have concluded that the ends of the earth would have to be some type of land to hold the water from all of it falling off the edge. Thus, perhaps not be frightened of falling off the edge and only concerned about how far and how long a trip it might be so they could be prepared with enough food, water, and supplies to last them.

Obviously, they had some higher brain functions if they knew how to create safe vessels for traveling the oceans and to craft tools for different jobs.