Why an ark?
In order to save Noach and his family from the impending Flood, God told Noach: "Make for yourself an ark (tevah) of gopher wood" (Bereshit 6:14).
Why an ark and not a boat or a ship? To help answer this question we must first find out what an ark is and how it differs to a boat or a ship.
The only other instance of an ark in the Bible, is the ark built by Yocheved for her son Moshe, when she placed him on the River Nile. (NB. We cannot compare this ark to the Ark of the Covenant as the Hebrew word for Ark in that instance is aron not tevah).
The common feature between both Noach's and Moshe's arks was that they were intended for refuge and not travel. They were not built to get them from point A to point B, but only to give them protection from the waters. Therefore, neither ark had any sails, oars, rudders or any navigational system whatsoever.
In essence, Noach had no control of the Ark. The Ark went wherever the waters took it. "When the waters increased, they picked up the ark, and it was lifted up above the earth" (ibid 17:7). "When the waters strengthened and increased upon the earth, the ark travelled along the face of the water" (ibid 18). "...The waters decreased and the ark rested...on the mountains of Ararat" (ibid 8:3-4).
Unlike the hero recorded in other traditons such as the Gilgamesh Flood epic, Noach could not steer the ark. He did not decide when to leave, in which direction to go and where to land the ark. He had an ark not a boat. Noach and humanity itself, was totally dependent on God for his security, safety and survival. By building an ark and not a boat, Noach submitted himself into God's care and trusted in His salvation.
Incidentally, Noach unlike the survivor from other flood traditions, is saved not because he was strong and wise, or because he was a descendent of a god and neither because of a fortunate chance. He is saved because: "Noach was in his generations a man righteous and whole-hearted; Noach walked with God" (ibid 6:9).
Once again, the Bible is introducing the revolutionary idea of Ethical monotheism in a pagan world that lives by the survival of the fittest.