I'd say around Mossdeep City where you could use dive and surf.
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If you use dive in dark patches of water then go in the seaweed then you can catch chinchou.
you can catch water Pokemon by surfing on water and by using all the rods
At victory road............ in the water....
Rectha
You can capture wailmer in ruby, sapphire and emerald on most routes where water is you can find either by using a rod or by using surf.