It was a slow rising flood, so millions of people were able to prepare and survive. During the time of this great flood, what remained of society lived on rusting away cruise ships, barges and makeshift boats and rafts. Many people enabled their houses to float, but violent ocean storms and nature has long since destroyed those.
It is now 30 years into the aftermath of the historical rain and floating islands that began to appear during the rain are becoming habitable. Pirates are the first to claim these islands and will sell them to the highest bidder. You’ve saved your entire life, sold all of your personal possessions and are ready for the chance to buy your very own island. It’s up to the island owners, and land owners upon that island, to rebuild a new land dwelling society.
After building a strong community, and defenses, battle other islands for their community resources. Islands may choose to opt out of direct battles, but will still passively compete for “best of” titles for game bonuses and resources.