For community spaces or internal living space? We have lots of ideas around here. Some based on experience and research, some on complete flights of fancy.
For internal living space design parameters, start with health and safety. The Ship Classification societies have published minimum standards for off-shore accommodations. (There are more updated versions available).
Additionally consider that the ocean environment has its own natural ecosystem stuff, but we can't natively participate in it to any great extent. We require equipment and accommodation to live there, so effectively, it is entirely "built" environment. It will need some planning to emulate natural environment and give people the beneficial psychological effect that natural environment conveys.
See also: http://seasteadingorg.wpengine.com/wp-content/uploads/2015/12/Feb2011_Report_p1.pdf