They're around Pacifidlog Town. I believe you use a Super Rod to catch it.
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You can't get horsea in xd.
Horsea is a Kanto region pokemon.
Use a good rod where you can find horsea use your pokedex if you seen one already.
King's Rock (if held) can occasionally make a pokemon flinch when the user attacks the foe. Certain pokemon will evolve from the item if those pokemon are traded. Horsea however is not one of those pokemon.
Altaria and Shelgon will make Horsea learn dragon breath. This move can also be learned leveling up in the Pokemon games.