Taken literally, the inference would be that the Lord God unmade what he created on the third day of creation, that is, the gathering of water into one place (the sea) so that dry land might appear. John himself as he wrote this was on Patmos in the Aegean Sea. The picture is of the sea overwhelming these (not so large) portions of land as a result of the earthquake. In keeping with the reference to cities in Revelation 16:19, we ought not to take the “fleeing” of every island literally.
20 And every island fled away, and no mountains were to be found.