This is my floating "Treasure Island" for the #MakeItFloat Challenge. It is designed to allow burying a treasure (coins) inside !

Video : https://youtu.be/ma-QDHCmzo8

Experiments:
My first design was a simple hemisphere (like a bowl) and coins attached at the bottom (I wanted it to be as simple as possible). It was a total fail and I learned two things for floating/ partially submersible objects :

• the air must be captured inside the object and should have no way to get out under pressure (the hemisphere was not stable and when the air began to get out it returned and sank instantaneously)
• All the added weight (coins) shall be placed on the vertical axis where the gravitiy center is located for best stability.
  My final design is stable and doesn't sink ;-)

Technical specifications :
Designed with OpenSCAD (http://www.openscad.org/)
Filament used for printing : PLA 1.75mm (yellow, black and green)
Printer : Prusa I3 Hephestos
Coin size : all currency will fit up to 32mm diameter
Maximum treasure weight for floating : 80g (not stable if heavier)
Island weight (without treasure) : 100g

Tips:

• You can place any kind of treasure inside the island, it is not limited to coins (but make sure it is waterproof if you want to put the island into water)
• The island can also be used for hiding objects (such as keys, usb drive, ...) as it looks like a simple decorative object when placed on a shelf.