A select few can be found out of the water, the most obvious of which being the Squirtle you can obtain from the Officer Jenny in Vermilion City after defeating Lt. Surge (Yellow only). You can also encounter Water-Types by Surfing on the water, but no matter what you do, there are some Pokémon that can only be caught with Rods.
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you can catch water Pokemon by surfing on water and by using all the rods
You can't catch wild Poliwhirl. You catch Poliwag though. Evolve your cute Poliwag into Poliwhirl by using a water stone. You can probably buy water stones in the department store in Celadon City.
All you do is Migrate a water Pokemon from your GBA game and catch it in the pond or the ocean. ( That is in Pal Park.)
In the water
Just use any rod on any body of water (as long as it's not a puddle!), and you'll catch water pokemon.