First Wave by Elizabeth Hopkinson

It was the gills that put people off.  Or so they said.  The media had been full of excitement when Atlantis first resurfaced.  Scholars enthused about ancient treasures.  Presidents and premiers competed to be the first to extend diplomacy.
Then the first migrants arrived.  Barefoot.  Dressed like something from Homer.  While the fledgling government demanded UN recognition and the right to carry the traditional trident, Europeans refused to have web-footed families polluting their reservoirs.
Of course, it was Atlantean workers who built the revolutionary Flood Pods that saved the south of England from annihilation, but people were quick to forget.