How do you get to seventh gym in pokeon emerald

You have to use the HM Surf to get to Mossdeep City.
after you beat team magma or aqua in the secret hideout in lilycove city on the water there used to be wailmer blocking ur way after u beat team magma or aqua surf past the wailmer and keep on going right you will end up in the townm and there is the 7th gym

note..... you can get master ball in the hideout :)
it is in mossdeep city