I believe that the "beast coming up out of the sea" is figurative, i.e. not referring to the literal ocean. According to scripture, the Abyss gk (abussos) is a place under the earth. For example:
"The fifth angel sounded his trumpet, and I saw a star that had fallen from the sky to the earth. The star was given the key to the shaft of the Abyss. When he opened the Abyss, smoke rose from it like the smoke from a gigantic furnace. The sun and sky were darkened by the smoke from the Abyss. And out of the smoke locusts
came down on the earth and were given power like that of scorpions of the earth.
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"They had as king over them the angel of
the Abyss, whose name in Hebrew is Abaddon and in Greek is Apollyon (that is, Destroyer)."
"The beast, which you saw, once was, now is not, and yet will come up out of
the Abyss and go to his destruction.
"“What is your name?” Jesus asked.
“Legion,” he replied, because many demons had gone into him. And the demons kept begging Jesus not to order them to go into
the Abyss"
"And I saw an angel coming down out of heaven, having the key to
the Abyss and holding in his hand a great chain. He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years. He threw him into
the Abyss, and locked and sealed it over him, to keep him from deceiving the nations anymore until the thousand years were ended.
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Strong's Concordance
abussos: boundless, bottomless
Original Word: ἄβυσσος, ου, ἡ
Part of Speech: Noun, Feminine
Transliteration: abussos
Phonetic Spelling: (ab'-us-sos)
Definition: boundless, bottomless
Usage: the abyss, unfathomable depth, an especially Jewish conception, the home of the dead and of evil spirits.
NAS Exhaustive Concordance
Word Origin
from
alpha (as a neg. prefix) and bussos =
buthos
Definition
boundless, bottomless
NASB Translation
abyss (7), bottomless (2).
The sea can be and is referred to as being Abyssal in literature because of its depths. However, in scripture it appears to be a place under the depths of the earth, which is usually referred to as a place where demons are imprisoned.