A tile is... huh, idk how to describe it. It's where you can walk. You can access hundreds of tiles with surfing. On ROUTE 119, where its raining and the huge river going through the whole route, outside Fortree City. Only 6 out of those hundreds can hold a feebas. There literally isn't a way to figure out what tiles they are. You have to fish about 3 times in each tile and if there's no feebas, you move over one step/tile and try again. This can take hours. There's no way to make this easy. You just need luck. Sorry.