These are all the secret or hidden achievements:

1: The Renovator!
How to unlock the secret or hidden achievement: unlock all crafting recipes from the Decoration Packages

Tip: hooking in Barrels.

2: Boxed In!
How to unlock the secret or hidden achievement: Cargo Bay Area > visit a hidden room directly under where the crane is operated

Tip: make a route through the boxes or revisit the room when it is flooded by water. If you have any doubts you can also watch this video guide.

3: You Should Not Be Here!
How to unlock the secret or hidden achievement: reach the -20th floor on Tangaroa

Tip: hidden button inside the Plantation area elevator > small, round button in the top-left of the elevator door-frame. If you have any doubts you can also watch this video guide.

4: Instrumentalist!
How to unlock the secret or hidden achievement: playing on the Piano

Tip: Tangaroa vending machines > 8x Tangaroa Tokens.

5: Former Glory!
How to unlock the secret or hidden achievement: find four Tiki Pieces whilst Metal Detecting

Tip: place them top of each other > correct order according to the colors on their chin and forehead.

6: Artistic Collection!
How to unlock the secret or hidden achievement: find all eight Developer Paintings

Tip: Metal Detecting for treasure.

7: An Ocean Cemetery!
How to unlock the secret or hidden achievement: Die 25 times

Tip: Use a new save file.

8: Not a great landing! and O Captain! My Captain!?
How to unlock the secret or hidden achievement: find the matching rare, small islands, and visit their main atractions, the plane and the boat respectively

Tip: start a new game on Easy Mode, avoid building a Receiver, and wait until the rare small islands spawn and you can visit their main atractions.

9: Is there a Utopia?
How to unlock the secret or hidden achievement: Find the note at the top of the Radio Tower

Tip: To reach the tower install and use a receiver and also x3 Antennas.

10: Exploring the depths!
How to unlock the secret or hidden achievement: Dive 100m below the surface

Tip: anchor before diving.

If you are interested in more guides like this make sure to CLICK HERE to check them out!