Underwater near sootopolis city.
you can get them by diving near route, umm i forgot but you can get them diving
route- 124, and 126. you have to be underwater though
route 129
Route 114
Use your pokenav to find it.
at route 128
Route 115
You can use the Good Rod in Pokemon Sapphire to catch Pokemon. To get it, talk to a fisherman on Route 118.
Slakoth - Petalburgwoods Gulpin - Route 110
Marill is a water type of Pokemon which is featured in the Pokemon anime. It is not possible to find one on route 104 on Pokemon sapphire.
Wailord is an immense Water type Pokemon. It can be encountered in the wild in Pokemon Sapphire while Surfing on Route 129.
you can find a feebas in route 118 and 119 but its hard to find