I am thinking of a seastead design that would need like 12 feet of water depth.
I know some sailers are wanting boats that draw less than 4 feet.
I know many cruising areas like the Bahamas this would not work.   But would
there still be plenty of areas one could go in the Caribbean, Mediteranian,
and South Pacific with this draft?  How restrictive will
this feel in practice? 

The seastead design is so stable it does not need to be inside a harbor.
So if much of the issue is harbors, then maybe it is not so much of a problem.
The seastead will park with Dynamic Positioning (DP) as it has lots of solar
power and huge propellers for very efficient slow movement, so it will not
need to anchor in deep water.