Where to catch Tentacool

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

• Coastal Biome — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Abandoned Ship — Good Rod, Old Rod, Super Rod, Surf
• Dewford Town — Good Rod, Old Rod, Surf
• Ever Grande City — Good Rod, Old Rod, Surf
• Lilycove City — Good Rod, Old Rod, Surf
• Mossdeep City — Good Rod, Old Rod, Surf
• Pacifidlog Town — Good Rod, Old Rod, Surf
• Route 103 — Good Rod, Old Rod, Surf
• Route 105 — Good Rod, Old Rod, Surf
• Route 106 — Good Rod, Old Rod, Surf
• Route 107 — Good Rod, Old Rod, Surf
• Route 108 — Good Rod, Old Rod, Surf
• Route 109 — Good Rod, Old Rod, Surf
• Route 110 — Good Rod, Old Rod, Surf
• Route 115 — Good Rod, Old Rod, Surf
• Route 118 — Good Rod, Old Rod, Surf
• Route 119 — Good Rod, Old Rod, Surf
• Route 121 — Good Rod, Old Rod, Surf
• Route 122 — Good Rod, Old Rod, Surf
• Route 123 — Good Rod, Old Rod, Surf
• Route 124 — Good Rod, Old Rod, Surf
• Route 125 — Good Rod, Old Rod, Surf
• Route 126 — Good Rod, Old Rod, Surf
• Route 127 — Good Rod, Old Rod, Surf
• Route 128 — Old Rod, Surf
• Route 129 — Good Rod, Old Rod, Surf
• Route 130 — Good Rod, Old Rod, Surf
• Route 131 — Good Rod, Old Rod, Surf
• Route 132 — Good Rod, Old Rod, Surf
• Route 133 — Good Rod, Old Rod, Surf
• Route 134 — Good Rod, Old Rod, Surf
• Seafloor Cavern — Good Rod, Old Rod, Surf
• Shoal Cave — Good Rod
• Slateport City — Good Rod, Old Rod, Surf
• Sootopolis City — Old Rod

In Johto & Kanto (Gold, Silver & Crystal)

• Cherrygrove City — Good Rod, Surf, Walk
• Cianwood City — Surf
• Cinnabar Island — Good Rod, Old Rod, Surf
• New Bark Town — Good Rod, Old Rod, Surf
• Olivine City — Good Rod, Surf
• Pallet Town — Good Rod, Old Rod, Surf
• Route 12 — Good Rod, Old Rod, Super Rod, Surf
• Route 13 — Good Rod, Old Rod, Super Rod, Surf
• Route 26 — Good Rod, Old Rod, Surf
• Route 27 — Good Rod, Old Rod, Surf
• Route 32 — Good Rod, Old Rod, Super Rod, Surf
• Route 34 — Surf
• Sea Route 19 — Surf
• Sea Route 20 — Good Rod, Old Rod, Surf
• Sea Route 21 — Good Rod, Old Rod, Surf
• Sea Route 40 — Surf
• Sea Route 41 — Good Rod, Old Rod, Surf
• Union Cave — Surf
• Vermilion City — Good Rod, Old Rod, Surf
• Whirl Islands — Surf

In Kanto (Red, Blue & Yellow)

• Cinnabar Island — Super Rod, Surf
• Pallet Town — Super Rod
• Route 11 — Super Rod
• Route 12 — Super Rod
• Route 13 — Super Rod
• Route 17 — Super Rod
• Route 18 — Super Rod
• Sea Route 19 — Surf, Super Rod
• Sea Route 20 — Surf, Super Rod
• Sea Route 21 — Surf, Super Rod
• Seafoam Islands — Surf
• Vermilion City — Super Rod
• Viridian City — Super Rod, Surf

Tentacool — full Pokédex entry