This is a Keychain that will float if you drop your keys in the water. Depending on the watertightess of your prints, this will not float forever, but will keep it afloat long enough to retreive your keys. I used 3 perimeters, four is probably better.

The buoyancy of this keychain is not infinite. Having a couple of dozens of keys on there will very likely sink this, so test beforehand. To float, it needs air inside, so do not fill it up with plastic. 10 % infill is plenty.

One model is a plain keychain, the second has the logo and the name of Chantiers Amel on it. 

Footnote: The name Amel and the logo is very likely a trademark of Chantiers Amel, France.