Augustine of Hippo

Having concluded his prophecy of the judgment awaiting bad people, John has to speak of what is to befall the good.… [The new heaven and earth] will happen in the order which he indicated, by anticipation, in the earlier verse where he said he saw sitting on a throne one from whose face heaven and earth fled away. First, to be sure, will come the judgment of those uninscribed in the Book of Life and their consignment to eternal fire.… Afterwards, this world as we see it will pass away, burned away by terrestrial fires, just as the flood was caused by the overflowing of terrestrial waters. This conflagration will utterly burn away the corruptible characteristics proper to corruptible bodies, as such; whereupon our substance will possess only those qualities that are consistent with bodies immortalized in this marvelous transformation—to this end, that the world, remade into something better, will become fit for people now remade, even in their bodies, into something better.

It is hard to know whether [the sea] will be dried up by the terrible heat of those flames or will itself be transformed into something better. For though we read that there will be a new heaven and earth, I cannot recall having ever seen mentioned a new sea, save perhaps in that verse of the Apocalypse, "a sea of glass similar to crystal." Yet in that passage, John was not talking about the end of the world; moreover, he did not claim to have seen a sea proper, but something like a sea. Still, as prophecy is prone to intermingle the literal and metaphorical and so veil its meaning, it may be that in our present text, "and the sea is no more," John was speaking of the identical sea he spoke of earlier: "And the sea gave up the dead that were in it." For then, this world of ours, made restless and stormy by the lives of men (and, hence, figuratively, called the sea), will have passed away.