The OSM coast line is not the National Park boundary. 

"The natural=coastline tag is used to mark the mean high water springs line along the coastline at the edge of the sea. "

See https://wiki.openstreetmap.org/wiki/Tag:natural=coastline

The National Park boundary is created by the government .. it could be anything... such as the low water mark... but it is unlikely to be the OSM coast line. The original source of the entered National Park boundary comes from the Government! Leave it alone!
See https://www.openstreetmap.org/changeset/72634280