I think that for now we can only speak of urban design for very small seasteading villages or settlements.
So, instead of dealing with larger scale of groups of buildings, streets and public spaces, whole neighborhoods and districts and entire cities with the goal of making urban areas functional, attractive, and sustainable, the task at hand is much, much simpler.
Even so, I do believe that the urban design theory as an inter-disciplinary subject that utilizes elements of architecture, real estate development, economics, political economy and social theory, making connections between people and places, nature, environmental stewardship, social equity and economic viability into the creation of places with distinct beauty and identity, etc. will stay more or less the same.
But, the main difference is that the seasteading "canvas" is floating. Therefore, all "weight" (buildings, water, waste and fuel tanks, generators, public spaces, etc.) HAVE to be evenly distributed on the seastead according to what in naval architecture is called ship stability.
Otherwise, the seastead will be floating heeled at all times (at best) or possibly capsize (at worst)
Therefore, seasteading urban design will be, more or less, quite "symmetrical" in nature.