In Pokémon Ruby, both the original and alpha remake, route 128 is located in the southeast of the Hoen map. It is an ocean route and can therefore only be accessed by using surf. When accessing the route for the first time, you must enter from route 127. However, once the map has been fully explored, the route has three main access points.
HM03 Surf, which is required to enter route 128 and other connecting routes, can be found in Wally's house in Petalburg City after you've defeated the Petalburg gym leader. After you've finished exploring route 127 and the Underwater Trench area for the first time, head south to enter route 128.
At first, route 128 runs north to south, but eventually it splits into two separate paths after the Seafloor Cavern. One path runs north to south and the other runs east to west. By following the route all the way from north to south, you can enter route 129, another ocean route. By following the east to west path, you reach Ever Grande city, the access point for Victory road and the Elite Four. Route 128 can be accessed from both Ever Grande and route 129 once both locations have been discovered.
If you have a Pokémon using the dive Hoen map, route 128 also connects to route 127 via an underwater passage.