http://www.openstreetmap.org/?lat=47.3717&lon=-0.7956&zoom=13
The island is here under water 
How can we fix that …
I see that the island and lake have both been edited today, the tagging looks good and if you switch to the Osmarender layer you can already see the island appearing.
So I think it will render OK the next time the Mapnik layer is rendered.
Right now the island seem to have tags
name=La Loire
natural=water
source=Dshpak_landsat_lakes
waterway=riverbank
In addition it is an inner ring member in a multipolygon relation “La Loire”
Should this feature with these tags really be rendered as an island? If I can read the tag natural=water is suggesting that it is a lake. Without natural=water or any other tags this feature would be rendered as it were land, as a hole in the La Loire relation and coloured with the background colour. But isn’t this island an object as itself and not just a hole in a river and thus close to nothing? Place=island souds somehow suitable, or natural=land, but I don’t know. Tricky tagging may be one reason why the OSM project seems to be in stagnation phase when it comes to user inputs.