The Ark is not supposed to sink, so hull integrity and stability are essential. It is prudent to ensure a safe margin in any sea state likely to be encountered, especially considering the magnitude of the floodwaters and the (presumed) novelty of the vessel. Both these factors would drive the project to use a high safety factor - to err on the conservative side. The job had to be done thoroughly.
Watertightness (E) The ship must not leak excessively, or take water onboard. A sound hull, storm hatches etc
Strength (E) The ship must not break in half or flex in rough seas. Wave bending moment, wave slamming, torsion etc.
Stability (E) The ship must not capsize - a combination of roll stability and broaching avoidance.
Seakindliness (D) The ship motions should easy on its crew and gear. Accelerations, angle of roll etc
Essential needs met (E) The ship must meet the essential needs of cargo and crew during the voyage, Space, ventilation etc
Risks Managed (D) Fire hazards, access and handling issues, repairs etc.
Buildable (E) It must be possible to construct the ship using available technology. Wood construction, manual labor etc
Costs Optimized (D) The ship should require a minimum of materials and labor without jeopardizing any of the essential criteria.
(E) = Essential, (D) = Desirable.