Didn't know that about Ikaris. But there's some of your survivors. There were others. There's another account in Namor 62 that Ray referenced, that says Atlantis only partially sank??? so that one end of it became Avalon and the other end of it became Attilan. Seriously. WTF? I mean, a tsunami that changes the entire map of the world, but the center of the explosion leaves two islands? Plus, there's at least one colony, and probably more that I am forgetting, that survived in a hidden city -- Netheria?
Does it talk about how the Ancients Paul Destine was looking for were really ... Atlanteans? Or Lemurians? That took the Serpent Crown to Antarctica and renamed themselves?
Oh, I found your Cassandra. The one who had early warning of the coming Cataclysm. It was a sorceress named Zhered-Na. She becomes part of the magical lore, writing / her writtngs in a Tome, and her descendant is Jennifer Kale. Of course air breather Kamuu and Zarta banished her, so she survived the destruction.