‘And before the throne as it were a glassy sea, like crystal.'
Compare the ‘firmament' carried by the cherubim which bore the throne of God in Ezekiel 1:22 which also was of crystal. This sea is based on the molten sea in the Temple of Solomon (1 Kings 7:23-39; 2 Chronicles 4:2-10). There it was a large bronze sea of over 16,000 gallons capacity (nearly 73,000 litres) mounted on twelve bronze oxen, and was for the priests to wash in (2 Chronicles 4:6). While the water would be as clean as they could get it, it would be fairly murky (we tend to forget they had no pure water supply on hand), and was for the removal of ‘earthiness'.
Here it is replaced by crystal-like glass, which is a symbol of unearthiness, holiness and purity. The washing for priests, like all Old Testament washings, removed the earthiness that was preparatory to waiting on God for cleansing. (Every mention of washing with water in the Pentateuch is followed by the phrase ‘and shall not be clean until the evening', thus it was preparatory not finally effective). Now in Heaven there is no more earthiness, all is pure, and therefore no sea for washing is required. Instead the crystal sea reflects the holiness and purity of God and of the redeemed. That is why the sea is now crystallised, a reminder of what is and what was.
The glassy sea is mentioned again in Revelation 15:2 where it is mingled with fire and those who have gained victory over the Beast gather there, holding harps of God, made pure through tribulation. They have washed their robes and made them white in the blood of the Lamb (Revelation 7:14). At this point they sing ‘the song of Moses (Deuteronomy 31:30; Deuteronomy 32:44), the servant of God, and the song of the Lamb' which is based on a number of Old Testament scriptures including Deuteronomy 32:3-4, and stresses that He is true, righteous and uniquely holy. This is also what the sea symbolises.