You have to talk to the 3rd gym leader after you beat your dad and he is in the middle on mavlle city.And he will give you the key!
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After a certain point in the game, you will see Watson standing in the middle of Mauville outside of his gym. If you talk to him and agree to help him with his problem, he will give you the key to New Mauville.
You need the Mauville badge, go to Mauville and fight the gym leader, but you must first fight Wally to get to the door open.
From Mauville City, go south, you'll meet the Fisherman. Surf and keep to east. Then, you reach the New Mauville.
first,talk to the gym leader at mauville(he will be out side the gym)then he will tell you about new mauville,then he will give you the basement key.second,you need to go to the river in front of the trick house and use surf and go forward.you will see a cave.go in and you are new mauville!
New Mauville MissionMr. Wattson wants you to turn off the big machine at the end of New Mauville by pressing a red button. The cave's full of Electric-type Pokemon and no Trainers.