Christians: Who is the sea beast of Rev. 13 and who are "they" that worship him. Rev.13 verse 4?

>> Christians: Who is the sea beast of Rev. 13 <<

Normally goes by the title "the Antichrist"

describes a world ruler who promotes worship of Satan (the author identifies the Dragon as Satan)

>> who are "they" that worship him <<

Everyone other than devout and stalwart Christians.

Probably, the International Monetary Fund and various, related entities, like the Federal Reserve.

The beast is a political creation among the masses of mankind. 

The Bible book of Daniel sheds much light on this prophecy (compare  Chapter 7) Daniel tells us that these beasts represent “kings,” or political kingdoms, that rule in succession over vast empires.​—Daniel 7:17, 23.

Regarding the beast of Revelation 13:1, 2, The Interpreter’s Dictionary of the Bible points out that it “combines in itself the joint characteristics of the four beasts of Daniel’s vision . . . Accordingly, this first beast [of Revelation] represents the combined forces of all political rule opposed to God in the world.” This observation is affirmed by Revelation 13:7, which says of the beast: “Authority was given it over every tribe and people and tongue and nation.” 

People worship to it in the meaning they rely on rulers more than on God.

At the close of the first century C.E., the resurrected Jesus gave the apostle John a series of stunning visions. (Rev. 1:1) In one of them, John saw the Devil, represented as a dragon, standing on the shore of a vast sea. (Read Revelation 13:1, 2.) John also saw a strange beast rise from that sea and receive great authority from the Devil. An angel later indicates to John that the seven heads of a scarlet beast, which is an image of the beast of Revelation 13:1, represent “seven kings,” or governments.

 At the time of John’s writing, five of those had fallen, one was currently in power, and one had “not yet arrived.” What is the identity of those kingdoms, or world powers? Let us consider each of the heads of the beast described in Revelation. We will also see how the writings of Daniel added detailed insights into many of these kingdoms, sometimes centuries before they came into existence.

The word 'sea' often referred to a multitude of people; In this case, the Roman military.  The Emperor wore the military uniform as described also in Rev., and was 'commander in chief', the 'beast'.

The Interpreter’s Dictionary of the Bible comments: “The first of these beasts [of Re 13] combines in itself the joint characteristics of the four beasts of Daniel’s vision . . . Accordingly, this first beast represents the combined forces of all political rule opposed to God in the world.”—Edited by G. Buttrick, 1962, Vol. 1, p. 369.

The dragon is always the devil. Who gives authority to the worldly governments who are represented by beasts.

(Revelation 13:4) And they worshipped the dragon because it gave the authority to the wild beast, and they worshipped the wild beast with the words: “Who is like the wild beast, and who can do battle with it?”

That would be the UN. Who has came out as the savior of the world and a representation of all governments. Who in the time of the end will destroy all religion. That time is fast approaching. www.jw.org

Most modern Biblical scholars believe that when the Book of Revelation speaks about the beast it was most likely referring to the Roman Emperor and the Roman Empire. We know this for a couple of reasons. (i) In prophetic speech, the image of a beast was used to symbolize dominant and oppressive powers. You see this especially in the Book of Daniel. (ii) Within the Apocalyptic literature of the time, Rome was referred to as a beast. When Pompei conquered Jerusalem in 64 B.C the imagery that was used to refer to the conquest was the imagery of a beast devouring a city. 

the sea beast is the Loch Ness monster