lol, nice one theres loads of stuff before the 8th badge unfortunately, theres a whole big thing with Groudon and Kyogre, and Rayquaza You'll need Dive to get into Sootpolis City, and you get Dive from Steven at his house on the island where you get the 7th badge. Once you have that you Surf south until you come across a ring of deep & shallow water, go to the southern part of the ring and dive and go into the cave, then surface by the submarine. This is team aqua's place so you'll run into heaps of them, until you reach the end with Archie and Kyogre, then battle away. Once thats done you'll have to head back north, then west halfway back to lilycove, then south to a big mountain thing sticking out of the water, go around to the bottom and dive, then enter the cave and up, and you're in Sootopoilis. Theres a bit where you have to go get Rayquaza and stop a fight or something, but I cant remember where that fits in, but once its all over you can challenge the gym leader.
starting from the island with the space rocket place, surf down and stay to the east side, and you'll come across a gap which leads to an island with fishermen and stuff, dont go there its down further in another gap and its wide, theres a couple of people standing on shallows and some swimmers etc, and theres a deep channel that runs along it. Follow it and it should lead to a water fall, at the top theres a pokemon center and victory road, the annoying thing is you need flash.