surf on water or get a fishing rod
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Poliwag can be caught by using any kind of rod in Route 43 (Pokémon Heart Gold / Soul Silver)
You can't catch a poliwag in Pokemon Sapphire. You have to trade from Pokemon Fire Red or Pokemon Leaf Green.
Pokewalker (blue lake) or at the safari zone on water or good rod.
I'm sorry, but you will have to trade from Pokemon LeafGreen or FireRed. There is no way to find a Poliwag in the wild in Emerald.
You'll need to trade for it.
use the water stone
Nope, you have to fish.
poliwag is usually on the beach.
You can find poliwag at: Silent Chasm 5-9, Northwind Field 1-4, Waterfall Pond 1-5 (8.1%)
Shiny Poliwag can be anywhere in water, but you would have a very very very little chance of finding it.
Nowhere, you need to breed Poliwhirl/wrath/toed.