Dewgong locations — where to catch

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

• Polar Biome — Walk, Static Encounter

In Sinnoh (Diamond, Pearl & Platinum)

• Sea Route 226 — Surf
• Sea Route 230 — Surf
• Victory Road — Surf, Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Seafoam Islands — Walk

In Kanto (FireRed & LeafGreen)

• Seafoam Islands — Surf, Walk

In Sevii Islands (FireRed & LeafGreen)

• Icefall Cave — Surf, Walk

In Kanto (Red, Blue & Yellow)

• Cinnabar Island — NPC Trade
• Seafoam Islands — Walk

Other games (6)

• Brilliant Diamond / Shining Pearl (Gen 8): Sea Route 226, Sea Route 230
• Let's Go, Pikachu! / Let's Go, Eevee! (Gen 7): Seafoam Islands
• Omega Ruby / Alpha Sapphire (Gen 6): Shoal Cave
• X / Y (Gen 6): Friend Safari
• Black 2 / White 2 (Gen 5): Giant Chasm, Seaside Cave
• Black / White (Gen 5): Giant Chasm

Dewgong — full Pokédex entry