Caravan 208B EX Glide ratio
You would have to ask someone in the engineering dept at Wipline for an actual number info@wipaire.com. 

I have enough float time to be willing to stick my neck out and make a somewhat educated guess. I recall from somewhere that the Caravan on wheels with a feathered prop has a really good L/D by single engine airplane standards, somewhere around 12.

Based on personal experience the glide of most seaplanes is pretty brick-like; I would say a good ballpark would be about a 1/3 to 1/2 reduction in L/D from the land plane.  So for a 208 on amphibs I'm guessing it's going to be between 6 and 8.  If you find out hard data from Wipline, let us know the actual number so I can find out how far off I am.

Fortunately, the PT-6 is so reliable the risk of a power off glide from the engine letting you down is pretty tiny.