You can't, you must have Surf.
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After you get Surf in Pokémon Ruby, you'll have to head to Fortree City which means you'll have to make it back to Mauville City first and then head on your way to Fortree City.
To do this you will need surf. # Go to the right of mauville city surf across the little passage # Get in the grass and go UP simple
OK, Once you get to Mauville City (if you know how to get there) surf across route 118 and head all the way up route 119 and on your right should be Fortree City.
it is close at the fortree city it is close at the fortree city it is close to fortree city
Go to Muaville city and surf right to route 118 and continue up to Fortree