It is odd that we don't look underwater for civilizations from before the end of the last ice age. We observe sea levels rising now from the melting of the ice caps, so why would it not have happened then too?
https://www.theguardian.com/science/2017/apr/26/the-first-br...
... which makes it clear that archaeologists are being pretty systematic about looking for settlements that were on the shore and are now drowned.