FAR 91.119 governs minimum safe altitudes. It contains a reference to open water. What qualifies as open water in the eyes of the FAA?
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: