Do we still use the term “seaworthy “ if a vessel traverses, say, the Great Lakes? Did the crew of the Edmond Fitzgerald deem her “seaworthy” before setting off?
A seaworthy vessel is a vessel that is considered properly equipped and fit for its intended use. I wouldn’t assign meaning beyond that, since the term “sea” is pretty broadly used. “Sea” just means a big body of water.
I made a “rowboat” once. Since I made it of plywood the curves were not what they should have been. While it was tested and “swimming pool worthy” I did not try to certify it as “pond worthy”.
Navies, or the ships that make them up, can be described as “blue-water”, “green-water”, or “brown-water”. A blue-water navy or ship is fully capable out in the open ocean, in deep water far from land. A green-water navy or ship is capable in the ocean, but only in the shallow waters close to shore. A brown-water navy or ship is restricted to rivers or other inland waters.