Where to catch Quagsire

In Kitakami (Scarlet & Violet – The Teal Mask)

• Chilling Waterhead — Walk
• Fellhorn Gorge — Walk
• Infernal Pass — Walk
• Kitakami Wilds — Walk
• Mossfell Confluence — Walk
• Oni Mountain — Walk
• Timeless Woods — Walk, Static Encounter

In Hoenn (Ruby, Sapphire & Emerald)

• Safari Zone — Surf

In Johto & Kanto (Gold, Silver & Crystal)

• Mt. Silver Cave — Walk
• Route 10 — Walk
• Route 12 — Surf
• Route 13 — Surf, Walk
• Route 14 — Walk, Surf
• Route 15 — Walk, Surf
• Route 26 — Walk, Surf
• Route 27 — Walk, Surf
• Route 32 — Surf
• Ruins of Alph — Surf
• Union Cave — Surf

Quagsire — full Pokédex entry