What does the literal oceans have to do with New Jerusalem coming down from Heaven to Earth?
If you first thought is "nothing" then you will understand how it represents the nations and their end!
Why would Jesus use an example of a mountain being cast into the sea?
Mark 11:23 For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith.
Rev 8:8 And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood
Which mount burns with fire? Sin-ai?
Mic 7:18 He will turn again, he will have compassion upon us; he will subdue our iniquities; and thou wilt cast all their sins into the depths of the sea.
Why are sins cast into the sea also?
Rev 21:1 And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.
Which is at the time of a new heaven and a new earth, or as Peter says
2 Peter 3:13 Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness.
Wherein dwelleth righteousness