Here's a rewording solutis versibus by Daniel Crespin:
Nondum abies ex montibus suis desecta descenderat in aquas fluidas, ut terras alienas adiret : et nulla littora nisi sua hominibus perspecta.
That probably clarifies a lot, especially that suis modifies montibus and that caesa modifies pinus, but here are a couple other notes.
Thomas Swinburne Carr writes:
Peregrinum—orbem, a foreign world or clime, as we speak of the "new world".
Pinus, the pine, of which ships were made. Hence, poetically, a ship.
Crespin says of orbem:
Orbem.] Pro orbis parte posuit, synechdochicè.
Here's my (beginner's) perspective on what's going on.
By interrupting the clause at caesa suis, Ovid is doing two things: (a) putting suis next to peregrinum to emphasize the contrast; and (b) creating "grammatical suspense", which emphasizes the modified nouns when you reach them.
So, here's a word-by-word translation that doesn't quite work even as poetic English, but I think maintains the sense and emphasis of the Latin:
Having not yet hewn from its native—a foreign world to survey—
mountains, a pine, and lowered it [as a ship] into the flowing waves,
mortals had not gotten to know any shores beyond their own.
I am taking the grammar to mean that the third line is the result of the first two. In the Latin, pinus is the subject of the first sentence, but clearly it's the people who are the agents of all the verbs and verbals: caesa, viseret, descenderat, and norant. The tense of descenderat is pluperfect, suggesting that it's referring to a time before something else. I take that something else to be the third line, which completes the thought—the whole sententia. The perfect and pluperfect of nosco refer literally to the process of getting to know, which is already accomplished at the time of interest—one "tense" later than the literal one.
A textbook from 1873 seems to agree:
norant] The pluperfect of this verb has the force of an imperfect.
What I think has happened is that Latin grammar is not as well suited as English to an absolute construction containing so many subclauses, so Ovid has exploited -que and the subtleties of Latin tenses to express the same meaning.