Saying 2 seas when you are referring to 2 actual seas is not a metaphor.
He let forth the two seas that meet together, (19)between them a barrier they do not overpass.
They meet via the river Jordan.
The problem with your explanation is that it is wrong. They do pass the 'barrier' just at a slow enough rate to create the impression that there is a 'barrier'.
He let forth the two seas that meet together, (19)between them a barrier they do not overpass. (20)O which of your Lord's bounties will you and you deny?(21)From them come forth the pearl and the coral.(22)O which of your Lord's bounties will you and you deny? (23)His too are the ships that run, raised up in the sea like land-marks. (24)O which of your Lord's bounties will you and you deny?
Why would a verse aimed at Arabs be referring to a little known lagoon in Indonesia? How does this count as a 'sign' to the audience?
So there is an explanation that relates to the actual context of the Middle East and is factually correct. There is another example that relates to a lagoon in Indonesia that is factually incorrect.
Strange that you prefer the 2nd option.