There is a master ball in the Team Magma's hideout in Pokémon Ruby Sapphire, and Emerald. Look in the Hoenn region for the Team Magma's base of operations.
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you get the master ball and leave then the wailmers wont be blocking the way to mossdeep. then you surf to mossdeep
after getting into team magmas hideout
the master ball is in team aquas hideout. When u find the room with 4 pokeballs the one on the top left (I think) is the master ball.
You get it in The Team Aqua Hideout by Lilycove City.
Master Ball which you can get at the (Team Magma or Team Aqua) hideout