At the Resort Area you will see this fat guy sitting in a pool. Talk to him and he will say something about a king of the sea or something. What he is really talking about is that if you fish at a certain part of the Resort Area, VERY strong Magikarp will appear. My highest catch was a lvl 69 Magikarp! My friend claimed to get a lvl 78 one! The certain area is on a bridge in the middle of the Resort Area! Catch him like a normal Magikarp! No Masterballs needed!
P.S This is true because it says it in THE POKEMON PLATINUM OFFICIAL GUIDE BOOK! So please don't put thumbs down on me saying that I am lieing! :(