The current build of How to Fish has four different islands and five locations for you to access. While the Lighthouse is the default location, you can unlock four more islands to explore inside the game.
In this guide, I will list down the key steps that will allow you to gain access to the new locations.
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How to unlock all islands in How to Fish
The first location, as I mentioned, is the Lighthouse and it comes unlocked by default. Once you kill the Spider Crab, our real adventure begins.
Island 1 (Forest)

Once you kill the Spider Crab, take the keys from the Lighthouse Keeper. Purchase the radar, and get on your boat. You’ll find a green dot appearing at approximately northwest from the Lighthouse.
This is precisely where Island 2 is located, and it’s essentially a mini swamp/forest.
Island 2 (Desert)

You’ll receive the coordinates for the second island once you defeat the Giant Piranha and feed the skeleton to the lady by the lake. The second Island is located to the west of the Forest island.
If you open your radar, you’ll locate it using yellow spot.
Island 3 (Rocks)

The third island is located to the west of the previous one. To get the coordinates, you’ll have to defeat the Puffer Fish, which is a pretty strong boss. It rolls after you and can do a lot of damage if you can’t dodge the incoming attacks.
Once you kill the Puffer Fish, take its fin to the tourist NPC, and he will hand you the coordinates for the Island 3. Look for a red dot on the radar.
Island 4 (Volcano)

The final island in the game can be accessed once you defeat the Albatross. This is the big bird that has terrorized the islanders for a long time. The Albatross is difficult to take down for two reasons.
Its poop attack can do a lot of damage if you’re not under any cover. The Albatross also flies at a distance, which makes it a really hard target. Once you feed its severed head to the NPC in the shop, you’ll gain access to the Volcano.
You can spot the Volcano by checking out a pink dot on the radar. It’s directly north from the previous island and hard to miss (all thanks to the volcano itself). The volcano is also the final location where you can complete the game and head to the mainland.