the easiest way is to go back and forth between two places and check your pokedex. once you have seen him once, his location in the pokedex will be what ever route he is on. its not as efficient as the the roaming Pokemon tracker on the poketch in diamond, pearl and platinum, but it works. after about 12 encounters he will be a Pokemon you can interact with in the sea spirits den. there you have a proper battle with him and get a chance at catching him. if you knock him out, go throught the Pokemon league again
hope I helped!!