Where to catch a waillord in Pokemon ruby?

You can't catch a Wailord in Pokemon Ruby.

Wailord was introduced in Generation III, but it was only available in the sequels, Pokemon Sapphire and Emerald. It is found in the Seafloor Cavern in Pokemon Sapphire and Emerald.

So, unfortunately, you can't catch a Wailord in your Pokemon Ruby game.