What would you argue the "sea" represents in the story? Why throw a mountain into the sea, using your example. Also, why repeat this same scenario in Revelation, linked directly to wrath and final judgement?
The sea represents a great body of people. This interpretation can be derived from Revelation 17.
"Then one of the seven angels who had the seven bowls came and said to me, "Come, I will show you the judgment of the great prostitute who is seated on many waters, And the angel said to me, "The waters that you saw, where the prostitute is seated, are peoples and multitudes and nations and languages." (Revelation 17:1, 15 ESV)
The nation of Israel was destroyed by the Romans and its people exiled as slaves, trophies, or colisseum entertainment, as recorded by Josephus.
Thus, as Jesus prophecied, the mountain was removed from its place and thrown into the sea. This was the prayer of the martyrs in Rev. 6.
"When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne. They cried out with a loud voice, "O Sovereign Lord, holy and true, how long before you will judge and avenge our blood on those who dwell on the earth?" Then they were each given a white robe and told to rest a little longer, until the number of their fellow servants and their brothers should be complete, who were to be killed as they themselves had been." (Revelation 6:9-11 ESV)
I hope this clears up my interpretation more. Have a good day!