Ok, I bet some people have had problems finding the Golden whale (Which you need to fight the last extra boss: Whitebeard) so I am here to tell the specific location.
Note: You may need to upgrade your rod to a super rod-then upgrade it more with Steel stones.
Ok, first you go to the fossil beach, go to the spot where you first found Zoro and Sanji at the beginning of the game. When your there, head to where that Brown rock is at a spot you have to dash-jump to. There's a fishing spot there, but before you fish, beat those bad guys forthey are quite annoying. Ok, I hope this helps.
Note: You must throw the fishing rod all the way Approx. 150ft. Good luck! (You will need it @_@)