Well if I remember correctly when I sailed over here on my ship we tore the ship apart and used it to cover our dwellings and most of the carpenters that came over on the ship were themselves ship builders who used ship lap siding to cover the sides of ships. Could it get any more obvious? They used a weather/water tight ship lap because it is what they knew and it could survive water, waves and storms at sea and was apparently very water tight. This is just my guess. As if you had watched the rebuilding of some of the shotgun homes in Mississippi you would have seen they still use old ships/boats to build their homes while using the same ship lap siding as they used the old clapboards off of the boats that landed in Mississippi when first settling there. They were ship builders and carpenters who built the ships and the homes and passed on their trade.