Seadra locations — where to catch

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

• Canyon Biome — Walk
• Coastal Biome — Walk, Static Encounter
• Polar Biome — Walk

In Sinnoh (Diamond, Pearl & Platinum)

• Sea Route 226 — Super Rod

In Johto & Kanto (HeartGold & SoulSilver)

• Seafoam Islands — Super Rod
• Whirl Islands — Super Rod, Surf

In Kanto (FireRed & LeafGreen)

• Cinnabar Island — Super Rod
• Pallet Town — Super Rod
• Sea Route 19 — Super Rod
• Sea Route 20 — Super Rod
• Sea Route 21 — Super Rod

In Sevii Islands (FireRed & LeafGreen)

• Bond Bridge — Super Rod
• Five Island — Super Rod
• Five Isle Meadow — Super Rod
• Green Path — Super Rod
• Icefall Cave — Super Rod
• Kindle Road — Super Rod
• Memorial Pillar — Super Rod
• One Island — Super Rod
• Outcast Island — Super Rod
• Resort Gorgeous — Super Rod
• Tanoby Ruins — Super Rod
• Trainer Tower — Super Rod
• Treasure Beach — Super Rod
• Water Labyrinth — Super Rod
• Water Path — Super Rod

In Johto & Kanto (Gold, Silver & Crystal)

• Whirl Islands — Super Rod, Surf

In Kanto (Red, Blue & Yellow)

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

Other games (7)

• Brilliant Diamond / Shining Pearl (Gen 8): Sea Route 226
• Sword / Shield — The Isle of Armor (Gen 8): Honeycalm Sea, Loop Lagoon, Max Lair
• Let's Go, Pikachu! / Let's Go, Eevee! (Gen 7): Kanto Route 11, Kanto Route 12, Kanto Route 13
• Omega Ruby / Alpha Sapphire (Gen 6): Route 130, Route 131, Route 132, Route 133, Route 134
• X / Y (Gen 6): Ambrette Town, Cyllage City
• Black 2 / White 2 (Gen 5): P2 Laboratory, Unova Route 17, Route 17, Unova Route 18, Route 18
• Black / White (Gen 5): P2 Laboratory, Unova Route 17, Route 17, Unova Route 18, Route 18

Seadra — full Pokédex entry