"Sky" and "sea" are words that often have poetic synonyms in languages. For a conlang of mine, I came up with a poetic word for "the sky at night full of stars", and yesterday I was pleasantly surprised to find Mandarin actually has a word for this: 星空 xīngkōng (literally "star-sky"). (Note that 空 kōng by itself is typically an adjective meaning "empty", with nounal "sky" as a secondary meaning. I've mosty seen 空 kōng when meaning 'sky'. The normal words for "sky" are 天 tiān and the compound 天空 tiānkōng.)
What are some poetic synonyms for "sky" and "sea" that you know? If you can provide any commentary on the words, that would be even better.
English of course has "the heavens". And I think I've seen "the deep" used for the sea, but obviously rather specifically the underwater parts of the oceans, without including the surface.
(Apparently, "the firmament" is a word in English. Has anybody seen it in poetry? Also, I'm not an avid reader of English poetry, but I've sometimes seen the sky getting called "the aether" in some contexts—any comments on this one?)
Spanish similarly has the plural los cielos, which has biblical connotations (unlike English "the heavens", as far as I can tell), possibly because in Spanish "the Kingdom of Heaven" is translated as el reino de los cielos. El firmamento is another synonym, largely poetic.
The usual Spanish poetic word for "the sea" is la mar, which is the common word for the sea but with the opposite gender (normally the word is masculine: el mar). There are also a couple other synonyms for the sea, more poetic and much less widely understood by native speakers, both borrowed straight from Latin (which borrowed them from Greek): piélago and ponto. I find piélago very interesting because it has the e > ie sound change, typical of words that evolved naturally from Latin into the language—could it be it used to be a more common word in the past? Latin pelagus was poetic only in Classical Latin proper, but it is commonly found in prose in the periods afterward!
Latin poetry used the Greek borrowings aether (aetheris/aetheros, hic) and aethra (aethrae, haec) for the sky. It also had the poetic expression sub dīvō/dīvum 'under the sky', using a nominalization of the adjective dīvus 'divine'. For the sea, it commonly used a word that usually meant 'even surface', aequor (aequoris, hoc)... besides the Greek borrowings pelagus (pelagī, hoc(!)) and pontus (pontī, hic).