Completely straight sides do not seem appropriate for a boat, and I expect will contribute to drag. Currently carving into the sides of the boat to achieve a rounded, streamlined shape. This is not easy as the foam is difficult to cut into. It might not help that I am using a sandwich knife, but it is what I have available. Such a sight confirms that a computer scientist is at work. It is also difficult to ensure symmetry, though trial and error is converging to a reasonable shape.
Arun noticed a possible issue with the foam. There is a semi-cut line down the middle is intended to make it easy to snap the foam into sections. I believe that widening the gap slightly and sealing it up with glue should be sufficient to strengthen that area.