thanks for your reply, yes I agree it's a tricky case. This wiki page: http://osm.wiki/Tag:natural=coastline says that the coastline should be aligned to the MHWS (https://en.wikipedia.org/wiki/Chart_datum#Mean_high_water_springs), so the coastline should technically be shifted to the other side of the marina - but at the same time, this feels like a great example of wetland=tidalflat - so maybe the way you've tagged it is already perfect? i'm not really sure, but i think what you've done now looks pretty good