I was wondering if Marill is normally found by surfing, by using fishing rods, via encounters while walking or by some other way.
I am specifically referring to OR/AS, but I don't know if the way that Marills are encountered changes from game to game or from one generation to another.
It is found in grass, during fishing and surfing.
Hope this helps!
your best bet on a marill is on route 117 (the one with the daycare). there it can be found in the water surfing or in the grass. they arent super common in the grass but they are while surfing.
hope I helped!!
Source: this site and my own knowledge
You can find it in any one of these locations in Pokemon OR/AS :
Route 102, 111, 114, 117, 120, 123, Petalburg City. Hope I helped :D