Can you breed a Skitty and a Wailord?

Yes, a Skitty can breed with a Wailord. However, this can only be done if one of the Pokémon is female, and the other is male.

In the Pokémon games, breeding (producing an egg) can occur only between Pokémon that are of the same Egg Group. Pokémon are split up into Egg Groups based on certain attributes and physical characteristics. Most of the time, this creates scenarios where only similar Pokémon can breed. Occasionally, however, an odd pairing of Pokémon makes its way into the same Egg Group, the most famous of which is Skitty and Wailord. Skitty and Wailord are both members of the Field Egg Group, which is mostly comprised of quadruped Pokémon.