PetrW, yes, the beasts are related. They are the creations of the Dragon, Satan the Devil. But I am wary to conflate the scriptures unless there's very good reason for it. There's a reason why "the sea" and "the abyss" is used. It is not the same thing. The Society does the same.
I believe, In the abyss = inactive. Out of the abyss = active. Here's what a dictionary says of the abyss:
The abyss (Hebrew: תְּהוֹם; Greek: ἄβυσσος) can be favourably compared to the realm of the dead. Paul contrasts “ascent into heaven” with “descent into the abyss,” but because Christ was there, the abyss should not be conceived as an evil or demonic realm (Rom. 10:7; cf. Luke 8:31). A star (or angel) has the key to unlock the abyss, unleashing a locust-scorpion plague to torment mankind (Rev. 1:20; 9:1-3). God also allows the beast to come up from the abyss (17:8, 9, 11; cf. 11:7, 8). Later Satan the Devil would be sealed in the abyss for a thousand years (20:1-3).
The following is what I wrote to the Society. They argued that Rutherford and Co. were the "two witnesses," between 1914-18. I argued that's impossible. Beast from the abyss did not exist as yet. They obviously didn't like my criticism for I was disfellowshipped for my trouble:
A first fundamental flaw of the Watchtower interpretation of the two witnesses is the fact that they distinguish between Beast from the abyss of Rev. 11:7 and the scarlet-colored wild beast of 17:8; 18:1.
Most agree 17:8 and 18:1 refer to the same beast. In addition, in 17:8 the beast “is about to ascend out of the abyss,” whereas 11:7 describes the beast “that ascends out of the abyss.” These two are one and the same, the abyss and the state of the beast’s ascension being the common denominators.
A second shortcoming in their reasoning, compounding their error, is their belief that 11:7 and 17:8 follow in chronological order, which is clearly not the case.
In a footnote of Revelation-Its Grand Climax At Hand! (p.167) we discover some special pleading in order to justify their belief. Here the emphasis is craftily placed on the Hebrew instead of the Greek:
The “abyss” (Greek abyssos; Hebrew, tehohm’) refers symbolically to a place of inactivity. (See Revelation 9:2) In a literal sense, however, it can also refer to the vast sea. (Ps. 71:20; 106:9; Jonah 2:5) The Hebrew word is often translated “watery deep.” Thus “the wild beast that ascends out of the abyss” can be identified with “the wild beast ascending out of the sea.”