Gestreckt auf wildes Kraut, an die bemooste See." - Andreas Gryphius, Einsamkeit
Does he mean a lake (dialect?) or is the lake mossy?
Especially in Bavaria, Switzerland and Austria, "Moos" is not only used for moss, but also to designate a boggy/swampy area (see also Grimm and Duden). In modern German, instead, you would use "Moor" in such a case generally. But there are still a lot of examples where "Moos" is used in toponyms, denoting not a mossy area, but a (initially) boggy one that had to be drained.
So, the setting doesn't describe an "old sea", but likely an area that's characterized by water holes and bog. No settler didn't try to drain the land to cultivate it yet, and I assume you wouldn't be very lucky trying to cast for fishes in such a place. This way, the boggy land/water mixture is a "wonderful" setting to describe a scenery far from any civilization.
Gryphius writes „die See“, not „der See“. In modern German, this would indicate that he talked about "the sea" instead of a lake. But regarding Gryphius' time and language, I think this detail isn't conclusive. According to Grimm (again), the grammatical gender wasn't as clear as in modern language. Grimm:
Subsequently, the Grimm dictionary offers a remarkable number of sources using „die See“ for a lake and „der See“ for the sea. So I think you can't rule out that Gryphius used „die See“ when talking about a lake or - maybe - a pond.
When trying to interpret
Looking for a quotable take,
When going speculative, I'd look at colour and texture of moss as qualities to compare the sea to. Large bodies of water in the northern hemisphere can appear at an almost bottle (or moss) like green in the special lighting when there's dark, nearly violet clouds of a storm overhead and the last rays of sun are still reaching the water. This could also match the reference to
Sources: Barocke Thematik in der Lyrik des Andreas Gryphius gave me a few ideas.
"Die See" is the sea. "Der See" means: the lake.
As Andreas Gryphius lived in the 17th century, some words may have slightly changed their meaning. I would translate
Poetry is no exact science, though. ;-)