This is a special keyholder that seems made on porpose for people like me... My job brings me most of the time in the sea and I'm one of those guys that always lose everything they can lose (even if I have to admit that, year after year, I'm improving in this field..).
Water-buoy is the first keyholder created for valuable objects that can fall into the sea. If this happens Water-buoy self-activates:
A balloon hidden inside Water-buoy start auto-inflating converting it in a floating buoy
According to the manufacturers this clever item can sustain up to 1 kilogram. Moreover it is equipped with a flashing light too, in this way you'll be able to find it even in the darkness.
This video explain it all:
If you are interested in it, this is the official site