from the advice of the #osm chat, i dropped the Ubuntu JOSM and reinstalled the latest, so i could get the modern eWMS plugin among other upgrades.

The announcement of a coming Mapping Party caused me to look at osm.wiki/Boston - it
looks like we have a coastline lean in Boston. Chris noted it depends on which renderer, it doesn't show up in the main view. However Coastline viewer http://tile.openstreetmap.nl/coastlines.html?lon=-71.0861&lat=42.3736&zoom=13&layers=B00T was flagging the Malden and Mystic Rivers behind the Amelia Earhart Dam. Seems to me that freshwater on the inland side of a dam with locks
should not be natural=coastline, so i am adding waterway=dam,lock=yes
to the Amelia Earhart Dam and converting the offending coastline
polygons to waterway=riverbank. This may not fix whatever the intrinsic
problem is - the polygons seem rightway round, clockwise wet, the other common issues on osm.wiki/index.php/Coastline_error_checker seem ok. Hopefully Peter's polygon adjustments will help if taking them off Coastline didn't.

While I was at it, I refined some of the PGS coast polys for Malden River and added islets.