How to catch mew on pokemon sapphire version.

you have to get dive first then go to route 128

1st,u need to have dive.go to route 128,and dive in.there will be a cave in there(ocean cavern)there,its Team Aqua's main base.u have to go in that loonngg maze and then to the boss of Team Aqua.after that...Team Magma's boss will come and take u out of that.later,Steven will come and take u to sootpolis city.u have to stop the fight between Gourdon & Kyogre.after that all finihes,sootpolis Gym door will be opened!