I woudn't say this was "easy", by the way. The only reason I noticed the river problems was because I was looking at Map Notes near Berwick, where I used to live. The validator showed me loads of erros on the Cuckmere river. We had the river way crossing the coastline several times, and the administrative boundary glued to the river area at many points: it still is at some points, because it was just too much to bother with last night. Also, paths and bridges were messed up. I'm guessing this is partly because of the historical origins of OSM.