Actually the hull is a three-dimensional shape and the shape of every dimension affects stability and performance in the water. Take the length. A hull that is essentially flat end to end (has no rocker) tracks better in the water. A hull which rises from the middle to the ends (has rocker) turns better but is somewhat slower and less faithful tracking.
The hull bottom can be one of three general shapes, flat, vee or rounded. Initial stability varies with that shape. Throw in a keel into the mix which adds directional stability. Then the shape of the sides, hard chine, rounded chine and tumblehome. They all affect secondary stability. None is bad, they just serve different purposes. And provide for different handling characteristics. Some are proper for general recreation and flat water, some are better for moving water where maneuverability is important, and some are best for whitewater where maneuverability is of chief importance. The flatwater canoe will handle the faster water but with a lot more work since it wants to track straight. The moving water canoe will handle flatwater but with more work to keep it on course. The whitewater canoe will be a lot of work on flatwater but be capable of quick directional changes required to achieve eddys, clear obstructions and handle waves. Clear as mud I know.
FYI, though not too apparent in my photos above, the Old Town Guide 169 has a flat bottom and keel. It is long and wide and is very stable, but a bear to turn quickly in whitewater. It is rated as a general recreation canoe. The Bell Morningstar, a David Yost design, has a shallow arch hull, differential rocker (more lift front than rear) and significant tumblehome. She is very maneuverable but still manageable on flatwater. The canoe I sold last year, the Blue Hole Sunburst, had significant rocker, rounded hull bottom and soft chines. It was a bear on flatwater but at home on twisty fast water where constant direction changes were required. It also had poor stability and depended on the canoeist's balance for staying upright.
Old Town Canoes has a much clearer explanation on their website. Take the time to read it for a better overview of hull dynamics.
http://www.oldtowncanoe.com/canoes/canoe_anatomy/