S.S. Aqua is a sea vessel that appears in Pokemon Ruby, Sapphire, and Emerald. It's not a Pokemon you can catch, and it's not present in Pokemon Liquid Crystal.
Pokemon Liquid Crystal is a fan-made hack of Pokemon Crystal, meaning it has its own unique set of Pokemon and mechanics. While it features many Pokemon from other generations, it doesn't include S.S. Aqua.
If you're looking to encounter certain Pokemon in Liquid Crystal, it's best to check the game's Pokedex or online resources that list available Pokemon and their locations.