Where to catch Horsea

In Blueberry Academy (Scarlet & Violet – The Indigo Disk)

• Canyon Biome — Walk
• Chargestone Cavern — Walk
• Coastal Biome — Walk
• Polar Biome — Walk
• Torchlit Labyrinth — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Pacifidlog Town — NPC Trade, Super Rod, Trade
• Route 132 — Super Rod
• Route 133 — Super Rod
• Route 134 — Super Rod

In Johto & Kanto (Gold, Silver & Crystal)

• Whirl Islands — Super Rod, Walk

In Kanto (Red, Blue & Yellow)

• Cinnabar Island — Super Rod
• Route 10 — Super Rod
• Route 11 — Super Rod
• Route 12 — Super Rod
• Route 13 — Super Rod
• Sea Route 19 — Super Rod
• Sea Route 20 — Super Rod
• Sea Route 21 — Super Rod
• Seafoam Islands — Super Rod
• Vermilion City — Super Rod

Horsea — full Pokédex entry