After Surfing across Route 118 and meeting Steven on the way, you reach Route 119, which features extra-tall grass.
In the extra-tall long grass, you can encounter Oddish, Zigzagoon, Linoone, Tropius, and (rarely) Kecleon. If you Surf here, you can encounter Tentacool, Wingull, and Pelipper. If you use the Old Rod, you can encounter Tentacool and Magikarp. If you use the Good Rood, you can encounter Tentacool, Magikarp, and Carvanha. If you use the Super Rod here after you get it, you will encounter Carvanha.
In addition, it is possible to encounter Feebas while fishing in this Route. However, there are only six spots in the water where Feebas can appear, and those spots can also have Tentacool, Magikarp, and Carvanha in them. The six Feebas locations will change if you change the Trendy Phrase in Dewford Town.
There is no way to know for certain which squares hold Feebas, but if you do find a Feebas square, you will encounter Feebas about 50% of the time there, so try fishing several times in each spot in the water. If you don't get any Feebas after several tries, check a different square, and repeat this process until you find Feebas.
There is tall grass in the southern part of the route that you can't ride your bike through. You can find Bug Catcher Kent, Bug Catcher Greg, Bug Maniac Donald, and Bug Maniac Taylor in the southern long grass. Their pokémon include Ninjask, Illumise, Volbeat, Silcoon, Wurmple, Cascoon, Beautifly, and Dustox, and are in the ranges of level 24 and level 27.
In the southern grass, you can find an Elixir and a Super Repel.
North of the southern long grass, you can Surf northwest across the water to find Fisherman Eugene in Ruby and Sapphire, or Fisherman Chris in Emerald. He has Magikarp, Tentacool, Feebas, and Carvanha. You can get a Zinc northwest of that Fisherman.
After you Surf back to the main route, you can go north and you will find a small patch of long grass. There are a couple of Berry trees north of the grass. If you have the Acro Bike, you can go northwest of the small patch of grass and bounce across the white bridge to reach an area with a hidden Calcium and a couple of Secret Base locations.
Otherwise, go northeast from the small patch of grass, walking up the rock ledges. At the top, you can battle Pokémon Ranger Catherine. She has a Gloom and Roselia. In Ruby and Sapphire they are level 27. In Emerald they are level 26.
Go west from her and cross the bridge. At the other side, go north. Pokémon Ranger Jackson is there. He has a Breloom (level 28 in Ruby and Sapphire, level 27 in Emerald).
There is some long grass to the north, and then the path splits. If you have the Mach Bike, you can go west and ride up the slope to quickly reach the Weather Institute. Otherwise, you have to go east through the long grass.
If you take the northwest path, you encounter Bird Keeper Phil. North of him, there is a Hyper Potion. North of that, there is the slope that you can ride up with the Mach Bike. After the slope, you find Bird Keeper Hugh.
If you went up the east path, you can find an Elixir in the grass. To the north, on the rock ledges, there is a strange dark rectangle. If you check on it, Ninja Boy Takashi battles you.
Go northwest and you discover the Weather Institute. To the east, Team Aqua/Magma Grunts are blocking the bridge, so you have to go into the Weather Institute.