So in this guide I'm going to show screenshots for the location of every tentacle in the game and give a brief description of how to get the tentacle. I'll be putting the tentacles in order of when you might come across them in a standard playthrough and state any items required to get the tentacle.
If you need help finding specific items, please look at my other guide:Solution for COLLECTOR in MinitTentacle 1: Top of Desert Ocean - No Requirements
One screen south of the sign maker camp (which is also two screens south and one screen west of the Desert Campsite) is the ocean side beach of the desert. The first tentacle is here.
Tentacle 2: Stand on a Crate in the Ocean - Coffee, Sword Thrower
There's a tentacle in the middle of the ocean that you can't reach. You have to travel from the screen you get the Press Pass on, push the crate into the ocean, and then use the Sword Thrower to hit the tentacle.
Tentacle 3: On the Way to Island House - Boatwood
While riding in the boat on the way to the Island House you'll pass by a tentacle. Make sure to be facing the correct way and be ready to hit it as you go by. You have a small window, otherwise you'll have to take another trip.
Tentacle 4: In Water Cooler in Sword Factory - Press Pass
Inside the Sword Factory there is a water cooler with a tentacle in it.
Tentacle 5: In Sewers - Flippers
Inside the sewers there is a room to the left that you can swim into. There's a shark in here, so you have to swim on the outside edges from box to box to get to the tentacle.
Tentacle 6: Lost Temple West - Flippers
Lost Temple West is where the octopus that wants all of the tentacles is. If you swim north you'll find another screen with a tentacle in it.
Tentacle 7: Treasure Island - Flippers
If you swim to the Treasure Island the Old Man by the Lighthouse talks about, then you will find this tentacle (along with a coin). From the Lighthouse swim one screen west and then swim south until you find the island with the chest with the coin in it.
Tentacle 8: Warps in Basement of Island House - Coffee, Flippers
In the warp area of the basement of the Island House, you can leave the screen via the south. When you travel down there you'll have to swim to the warps, go into the one on the right, warp left, push the crate into the water, and then hit the tentacle from the crate.