Apart form the Battle Island no there is no second map or extra islands.
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Actually scrap that,you need the old sea map to get there but you have to trade it with someone who has the old sea map on Pokemon ruby/sapphire
There is no town map. You might be referring to the Pokenav, which is Ruby, Sapphire and Emerald's version of the Town Map. You can get it at Rustboro City from the Devon Corporation's President.
you get a map card for your pokegear in the second town you visit in the game an old man gives it to you
One island Pokemon centre after you defeat elite four and the 2 rockets at the volcano and find the gem.
there is no town map, you get a pokenav though from the big company in the game around the second or third gym battle that'll work better than a town map. just play the game, you'll get it. there is olso one in every Pokemon center *edit* The Pokenav is given to you by Devon after you retrieve the Goods package from the Aqua grunt in the cave outside of Rustboro right after you battle Roxanne for the first badge. - AliisaK