What qualifies as "open water" with respect to FAR 91.119?
Most probably the same definition as a dictionary's:

open-water. An expanse of an ocean, sea, or large lake which is distant from shore and devoid of nearby islands or other obstructions.

I searched for a legal definition but I can't find one. It's like defining a sparsely populated area, it's hard to draw a line. So sticking to charts is best I guess.

You might want to search for maritime laws that define such boundaries, which are borrowed by aeronautical charts.

On an FAA chart it's depicted like this:



Source: FAA