I have now
* started the CC BY permission request with the authors
* performed a 'simplify way' calculation over night resulting in: 

1. -750000 nodes @ 0.5m precision, testing @ 0.25m precision showed much less effect, corresponding to the fact, that most nodes are 5 to 50 m apart, so in reasonable distances for the task of mapping the seagrass areas

2. doubling of the .osm file size
1 and 2 are contradicting, but I guess, the .osm file keeps the 'simplify way' change information, so, doubling information. Is there a way to consolidate this 'simplify way' change (erase the original from the .osm, keep the reduced data), to see the net effect? 

In no case do I want to double the data load and would like to confirm the effectivness of the 'simplify way' procedure @0.5m precision.

Regarding few pixellated areas: These originate from the dataset and in some cases its development from interpretation of satellite imagery by the researchers. It means, that it represents the original and expert 'best knowledge' so far. Hopefully the OSM nautical community with all their local perspectives will find interest in further refining this data with local (not satellite pixellated) information, once this becomes apparent by initial display of the existing seagrass information. Intentionally, the icon fill for seagrass areas is designed without border lines of the area, so that pixellated source data has less pixellated optical impact in its OpenSeaMap, OpenNauticalChart and OSMAND representations.