A gunwale is the very top edge of a boat's side. You might rest your paddle on your canoe's gunwale while you take a break in the middle of a pond.
Nearly all boats have a gunwale, from the reinforced strip around the edge of a canoe to a narrow canal boat's wider gunwale that allows people to walk around the center cabin. Originally, this edge was called a "gun ridge," a band strong enough to support the weapons that were used on a war ship. A wale in this context is a "plank," from the Old English walu, "ridge."