Full maps for all areas, created using the Apple Arcade version.
The World Map
Click to enlarge...
The Three Towns
Siren Island has three town hubs that you'll visit on your travels. Arena Town is where the majority of the action goes down, but don't forget to visit Tree Town and Armor Town as and when they become available — and regularly, because the shops there stock things that Arena Town does not.
Arena Town (Hub)
Don’t forget to stop in and chat to Sky periodically. The NPCs around town have got plenty to say depending on what chapter you’re in. The little kid to the right of the save room is particularly useful as he gives you funny info about your various fusion coin powers.
Tree Town (Hub)
Don’t forget to play the mask game to win the squid heart. You need to hit a mask every single shot to win it.
Armor Town (Hub)
Just like most metroidvanias, the island can be broken up into distinct biomes which you'll only be able to fully explore having unlocked various upgrades. They're in roughly the order you'll encounter them in game, but there will be lots of backtracking to 100%.
On the subject of 100%, cards DO NOT COUNT towards your percentage. Squids and nuggets do.
I’ve tried to give a rough indication of where areas are underwater or full of soft earth, requiring the frog or drill powers respectively. The graphics are indicative, not 100% to scale.
Spoilers and Hints for CH6/Endgame!
If you get to the final chapter of the game (chapter 6) and realise you missed some nuggets here which are now inaccessible due to the Tour Route taking off, talking to the three scientists at the entrances to the Tour Route (one near the entrance from Laboratory, one in the Service Area, and one in Mossy Biome) will give you the nuggets you missed.
Not sure what happens about the heart squid.
Deep Sea Domain
Each siren on the island has its own dungeon to complete. You'll need to explore and defeat the bosses in each of these to unlock access to the endgame.
There are THREE squid hearts in each labyrinth. These are tracked in the top right corner of the screen under your gems while in the dungeon (a feature that wasn’t in the original launch on Apple Arcade).
Labyrinth #1 - Water Lily's Den
Labyrinth #2 - Coral Mine
Labyrinth #3 - Steam Vent Lab
Labyrinth #4 - The Boiler
Labyrinth #5 - The Squid Pit
Labyrinth #6 - Flying Fortress