mucx
I accidentally noticed that the lagoon http://www.openstreetmap.org/relation/2962390 is separated from the sea by the boundary=land_area multipolygon http://www.openstreetmap.org/relation/4056532. However, on all availible aerial imagery in JOSM, there is no land visible here. 
How can this be fixed? Is it a good idea to add a water area like a few kilometers to the southwest of this marker that connects the lagoon and the sea? (http://www.openstreetmap.org/relation/6520955)
Or can the boundary/boundaries be shifted so that everything is lined up with the actual visible coastline? If so, would that be also valid for the nature reserve boundary? I cannot find a source for its outline but my guess would be that it would also match the coastline. Any idea from someone (maybe with local/coastline knowledge) would be great! Unfortunately, I'm not very experienced regarding "sea mapping"...