To mark the specific location of where a boat can travel on a body of water.
Boats are not necessarily allowed wherever they can physically get to on a waterway.
the tag applies to ways or areas. The proposed tag is:
<tag k="highway" v="waterway"/> <tag k="name" v="<name of the waterway>"/> (optional) <tag k="ref" v="<reference of the waterway>"/> (optional)
This can be added as a way, overlaid on top of an area (a lake for instance), or added as a sub-key to a canal, river, or other linear item
e.g. water=canal highway=waterway name=Trent And Mersey Canal
It can also be sued to define the edge of areas where vessels area allowed.
A dark blue dashed line