On the Seattle Sectional/Terminal charts water is depicted in dark blue and light blue. In some cases the distinction between the two is right in the middle of the waterway (ie the same 'body' of water is one half dark blue, one half light blue). The aeronautical chart users guide says this distinction is to distinguish between OPEN water and INLAND water.
How can the same body of water be both open water and inland water? How is the distinction determined?
Why do we care Open vs Inland..ie Why is there a distinction made between the two? Is there an official FAA source that talks about this?