I have actually been thinking a lot about this the last few years. My current Idea is why not:

Build and sell Luxury Yachts (Mini Cruise Ships) that are Catamarans and sit way above the water with a square Cabin. The cabin could be built almost like a submarine with a pressurized cabin and connections on all 4 sides.

The thinking behind this is if things got really bad everyone who owned one of these ships could meet somewhere in the open ocean and they could all interconnect together to form a city with an absolutely massive flat surface on top for landing airplanes or helicopters and because all the ships are built on Catamarans there is docking available all the way around for smaller boats, subs, ships, etc.

If these ships were also fully self-sufficient and required no Fossil Fuels (grew their own food, used all electric power from solar, wind, hydro) and had ample desalinization units on board they could literally survive on the ocean indefinitely without ever stepping foot on land. They could grow their own fruits, vegetables, herbs and spices; reclaim salt from the desalinization units for seasoning and preserving foods and catch all manner of sea life for additional food.