The 'Glassy Sea' is similar to the 'Brazen Sea' that stood before Solomon's Temple (1 Kings 7:23-45, and was in front of the Ark of the Covenant, the 'Mercy Seat' which was the earthly Throne of God in the Old Testament times.
The earthly Tabernacle set up by Moses with all its vessels and instruments of service and types of worship, was patterned after the 'Heavenly Tabernacle' (Hebrews 9:23).