Early Game:
* Route 38 (near Violet City): You can find Poliwhirl in the water here. It evolves into Poliwrath, which is a Water/Fighting type.
* Route 39 (near Ecruteak City): Chinchou can be found here. It evolves into Lanturn, which is a Water/Electric type.
* Route 40 (near Ecruteak City): You can find Goldeen and Seaking in the water.
Later Game:
* Lake of Rage (near Mahogany Town): This is the only place in the game to catch a Gyarados, a powerful Water/Flying type.
* Route 47 (near Blackthorn City): Mantine, a Water/Flying type, can be found in the water here.
Trading:
* You can trade with other players to obtain Water-type Pokémon. There are many other Water-type Pokémon in the game that aren't mentioned above.
Tips:
* Use a fishing rod: You can find fishing rods in various locations, including in the Goldenrod City Department Store.
* Repel: Use Repels to avoid encounters with unwanted Pokémon while fishing.
* Super Rod: You can find a Super Rod in the Ilex Forest, which allows you to catch more rare Pokémon.
* Surf: Once you get the Surf HM, you can access more water areas and encounter more Water-type Pokémon.
Good luck catching those Water-type Pokémon!