Mantine locations — where to catch

In Hisui (Legends: Arceus)

• Tranquility Cove — Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Sea Route 41 — Surf

In Sevii Islands (FireRed & LeafGreen)

• Tanoby Ruins — Surf
• Trainer Tower — Surf

In Johto & Kanto (Gold, Silver & Crystal)

• Sea Route 41 — Surf

Other games (6)

• Sword / Shield — The Isle of Armor (Gen 8): Fields of Honor, Loop Lagoon, Max Lair
• Sword / Shield (Gen 8): East Lake Axewell, Galar Route 9, Giant's Mirror, Lake of Outrage, North Lake Miloch, South Lake Miloch, West Lake Axewell
• Omega Ruby / Alpha Sapphire (Gen 6): Battle Resort
• Black 2 / White 2 (Gen 5): Undella Bay, Unova Route 21
• Black / White (Gen 5): Undella Bay, Undella Town
• Colosseum (Gen 3): Pyrite Bldg, Pyrite Cave

Mantine — full Pokédex entry