Follow the canyon to the sea was a mission in The Precursor Legacy that Jak and Daxter completed to receive a power cell. It is an unassigned mission where Jak found a power cell after following the river leading through the canyon.
The Forbidden Jungle area has a large canyon, with a river leading through it. When you cross the first bridge, walk to the left and you can jump down to follow the river. Be careful of babaks around the area and especially jungle fish in the water. Near the end of the river, you will have to enter a small rock tunnel which leads into the ocean. Out here is a lone islet with a power cell.