What do you mean by "sustainable"?
Sustainable refers to working toward being resource independent from a supply route. At first most things will be a hybrid approach, perhaps utilizing current resources on the host island or atoll.
The plan is to be energy independent, recycle our own waste, gather and store our own water, create our own food, etc.
While we won't be making our own computers or having herds of cattle for steaks, we hope to reach the point where someone could live on the seastead without the need of anything from outside of the seastead. Of course, for more pleasurable living we would rely on trade.
This has big implications for things like growing populations or refugees. During the Syrian refugee crisis there was a very rich man that wanted to spend millions to buy an island for the refugees. If we can create a sustainable floating island where anyone living there has all the resources they need to live, then it will solve a lot of the world's problems.
We acknowledge that food will be the most difficult starting out but the rest should be possible early on.