For an explanation of how earth often represents a specific kingdom addressed in a Bible passage while sea represents the nations foreign to it see the following link: In the Bible “Earth” Signifies the Specific Land Addressed While “Sea” Symbolizes Foreign Nations. Though not stated in the above article, there is an added layer of depth to this aquatic imagery in the Bible. Throughout the Bible words like flood, waters, sea and Abyss often appear to symbolize two complimentary things. These words generally represent foreign kingdoms and yet at the same time also point to the Abyss, the underworld of the dead, simultaneously.
In the Book of Revelation, the Abyss is not just the land of the dead (Luke 8:31; Romans 10:7; Revelation 9:11; 11:7; 17:8; 20:1, 3). Abyss also has added symbolic significance representing foreign nations as does words like sea, waters or flood (Revelation 17:15; Daniel 7; 9:26; 11:10, 40; Psalm 65:7; 144:7; Isaiah 8:7-8; 17:12; 60:5; Jeremiah 46:7-8; 47:1-2; 51:55-56; Ezekiel 26:3; Joel 2:9; Nahum 1:8). Thus when the beast is said to come up out of the Abyss in Revelation 11:7 and 17:8 this is more than just resurrection imagery pointing to the fact that the beast rises from the Abyss, the underworld of the dead. This aquatic imagery also conveys a symbolic message concerning the arrival of the beast embodied by Caesar Titus and the Roman Army to Israel from the sea, waters or Abyss which is Gentile Rome. See the commentaries on Revelation 11 and Revelation 17.
This sea/Abyss symbolism presents two complimentary messages. First, it presents the destruction and conquest of Israel, the earth, by Rome, the sea or Abyss, as a return to the precreation state of the earth in Genesis 1:1-3. Here Israel, the earth, is metaphorically pictured returning to the formless abyss or water world of Genesis 1:1-3 after its conquest and destruction by Rome, the sea. This return of the earth to its preformed state sets the stage for its recreation as the new earth of Revelation 21:1. Second, the fact that the Romans, the sea, are signified by the Abyss, the realm of the dead, is also appropriate in light of the fact that during the subjugation of Israel by Rome during the Jewish War this powerful army left so much death in its wake. Jeremiah 51:25, 42 is a prominent example of this symbolism. In these verses, mountain represents a city or kingdom as it does so often in the Bible since cities were often built upon mountains for military defense. It is also clear in these vs. that sea represents foreign conquerors and the Abyss simultaneously:
“I am against you, you destroying mountain [Babylon], you who destroy the whole earth,” declares the Lord. “I will stretch out my hand against you, roll you off the cliffs, and make you a burned-out mountain. . . . The sea will rise over Babylon; its roaring waves will cover her.”
Jeremiah 51:25, 42 was fulfilled in 539 B.C. at the fall of Babylon. When the city of Babylon fell at that time it was not because of a literal flood. Rather, Jeremiah 51:25, 42 was fulfilled when Babylon, the destroying mountain, was conquered by the Medes and Persians represented in v. 42 by the rising sea. Here foreign conquest and the resulting death toll are presented as the sea or Abyss swallowing Babylon in a flood.
This double meaning underlying these aquatic terms is also found elsewhere in the Bible such as Psalm 144:7: “Reach down your hand from on high; deliver me and rescue me from the mighty waters, from the hands of foreigners.” In Psalm 144:7, as is the case in much of Revelation, the sea or waters represent both foreigners or foreign nations and the Abyss, the realm of the dead. Psalm 144:7 intentionally sends two related messages. Here the Psalmist wishes to be rescued from foreign adversaries. This desire to be rescued from hostile foreign enemies is also a poetic plea to be rescued from death and may even be a prophecy concerning the resurrection of the dead. The Psalmist’s plea for help echoes the plea of the departed to be rescued from the waters of the Abyss, the dark under world of the dead, during the time of the resurrection. Thus, as mentioned often throughout this commentary, the fact that the Book of Revelation uses waters, sea and Abyss as symbols of both foreign nations and the spiritual underworld simultaneously is not unique to Revelation alone.