From Revelation 10, quoted in the last post, we read:
" And the angel which I saw stand upon the sea and upon the earth lifted up his hand to heaven,
 And sware by him that liveth for ever and ever, who created heaven, and the things that therein are, and the earth, and the things that therein are, and the sea, and the things which are therein, that there should be time no longer"
These verses certainly look like a prime source for Blake's monumental figure of The Sea of Time and Space. Like a cornerstone it announces the framework of Blake's myth of time and Eternity. His theology indicates that Everyman or Albion came down from Eternity to Beulah, a sort of crossroad connection Heaven and Earth. Falling asleep he divided into the four zoas, descended via the Northern Gate into mortality; he lapsed into Eternal Death (the absence of the Eternal in corporeal life), down into UIro, where our Innocence is replaced by Experience. We are on the long journey (described also by the Illiad and Odyssey and many other stories.)
He spent his alloted time there until his mortal death; at that point Eternal Death ended and he woke up to Eternal Life. The whole thing is the Circle of Destiny (Erdman 302-03). The Sea of Time and Space expresses it pictorially: Odysseus (or Apollo or Jesus or Luvah or Everyman or Albion or you and me) has come to dry land where the Southern Gate invites him to return from whence he came.
Hundreds of poem and hundreds of pictures express this truth in hundreds of different ways; they all point toward the same myth, the same fundamental Truth of life (and death).