"If you defeat Ganon, this world will vanish, and the
Triforce will wait for a new master."

Having corruption vanish is not the same as having an ocean
vanish. We SEE at the end of WW after the WW King dies
that the Great Sea is doin' just fine...still there as much as ever...
even continuing into PH. So, no I don't think the Great Sea
vanished when the WW King died