1st of all it's just like emerald you get 5 badges and go to mauvill and outside will be wattson and he will give you the key and WALA go get to go on a vacation to N_E_W M_A_U_V_I_L_L!!!!!!!!! =D
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From Mauville City, go south, you'll meet the Fisherman. Surf and keep to east. Then, you reach the New Mauville.
Go south until you are below and out of Mauville City, and Surf in the water until you find a cave: That;s New Mauville.
In Pokemon Sapphire when the player defeats Gym Leader Norman in Petalburg City, Wattson, the Gym Leader of Mauville City can be found at the intersection of Route 110 and Route 118. Upon talking to him, he will give the player the Basement Key to access New Mauville City.
I would say the one on the way to de-activate the generator at new mauville near the cycling rode in Ruby, Sapphire, and Emerald.
First beat the Mauville Gym leader, Wattson.