Where to catch Seadra

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

• Canyon Biome — Walk
• Chargestone Cavern — Walk
• Coastal Biome — Walk, Static Encounter
• Polar Biome — Walk
• Torchlit Labyrinth — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Whirl Islands — Surf, Super Rod, Walk

In Kanto (Red, Blue & Yellow)

• Cerulean Cave — Super Rod
• Route 12 — Super Rod
• Route 13 — Super Rod
• Route 23 — Super Rod
• Seafoam Islands — Super Rod

Seadra — full Pokédex entry