2 And [kai] I saw [eido] as it were [hos] a sea [thalassa] of glass [hualinos] mingled
[mignumi] with fire: [pur] and [kai] them that had
gotten the victory [nikao] over [ek] the beast, [therion] and [kai] over [ek] his [autos] image, [eikon] and [kai] over [ek] his [autos] mark, [charagma] and over [ek] the number [arithmos] of
his [autos] name, [onoma] stand [histemi] on [epi] the sea [thalassa] of
glass, [hualinos] having [echo] the harps [kithara] of God.
2 And I
saw, as it were, a sea of glass mixed with fire, and those who had come off
victorious from the beast and from his image and from the number of his name,
standing on the sea of glass, holding harps of God. NASB
There is a brief study of the sea of glass in Rev. 4:6. However we will again briefly review the use of the term 'sea' in the scriptures.
Generally the sea is used in the sense of the nations of man. Being somewhat tumultuous and turbulent at times, they represent the rebellious nature and unrest of humanity throughout history.
The sea in heaven is a quite different sea in that, in similar fashion, it represents the expanse of heaven and the perfect calm without any controversy or conflict. It is peace to the maximum and the absence of worry, suffering pain, conflict, and so forth.
And in our current verse, those who were martyred during the tribulation, due to the evil of the anti-christ and his policies, of whom the anti-christ presumed that he had won a great victory over believers due to their deaths, did in fact lose and great loss, and it was the martyred, the victims, that were victorious, even in death.
They were, or will be, in heaven, in perfect peace and harmony as suggested by the harps, while back on the earth turmoil and judgment and all things evil will be ongoing with no human solution in sight.
John is setting up a contrast between heaven and earth, where heaven and eternal life is achieved in death, while the world has nothing to offer but death and a continuation of death for those who reject Chirst.
The sea of glass is the perfect peace, achieved not through Gods judgment, but due to Gods justice, which is represented by the fire. Justice originates in heaven and is the basis of judgment, and judgment occurs, or will occur on the earth where evil resides.