Explore the beach was a mission in The Precursor Legacy that Jak and Daxter completed to obtain a power cell. It was an unassigned mission where Jak found a power cell after exploring the first half of the beach.
Upon entering Sentinel Beach simply continue walking along the beach. Be careful of sand worms submersed in sand pits and the lurker crabs roaming the sands. Once you have reached a scattered collection of strong boxes and treasure chests on the beach you will have to be careful of bombs being fired from a cannon. Avoid the bombs and any lurkers and the power cell can be seen straight ahead, boxed in from the sides by two small cliffs from the upper bank, with the larger cliff creating a solid background. A set of natural steps will be to the left of the entrance leading to the cell. There is one final sand worm in a pit in the small entrance, although you can avoid him by jumping on the higher plateau using the steps on the left and jumping down behind him.