One of the planets I am building has a very large crescent shaped continent that I want to say came about when a super continent split and the areas below sea level flooded. Could this have happend?
Not only could this have happened, but it has happened many times here on earth.
There is speculation about just how intense it was but the flooding of the Mediterranean basin here on earth could have been a cataclysmic event:
It's a specific instance of an Outburst Flood.
You just need to have appropriately placed fault lines in the continent that pull it apart in the shape you want to then flood and the eventual flooding can be as fast or slow as you want depending on whether you lower the middle and then let water in or do it the other way around.