Just a quick note on seats and slippery surfaces... when I built the seat frames as illustrated in CanoeCraft, I added on the wooden slats running sideways since they were the quickest, and I wanted to get out on the water. Of course, these were varnished to a high gloss and slippery. These did not feel safe at times, especially when the water was rough and a broadside wave would tilt the canoe, causing one to slip sideways sitting down and upsetting the balance. Needless to say, in cold water an upset can be serious and hypothermia has killed paddlers here in Canada.
Several days ago, I took out the slats and replaced them with rawhide lacing done snowshoe-style. The rawhide is rough and doesn't allow for any sideways slipping at all, giving a much more secure feeling in waves. Although I mostly paddle kneeling, every once in a while it's nice to sit up and stretch out - this is when an upset might happen if the attention wanders and the seating material is slippery, and a wave happens to tilt the canoe.