Right, I don't suppose you remember the small route with a dock on it, with absolutely nothing on it? No trainers or anything? Basically, it was surrounded by water...I think it is to the left of Hearthome City. If you have Surf taught to a Pokemon, you Surf off the top of the deck, and head left. You should eventually end up on a new route. Battle your way through, and you end up in Canalave City. I'd heal up though before you go over the bridge, cos your rival will fight you on the bridge.