The issue with homeless camps is the lack of square footage available for extended periods.

This is Obviously a massive simplification, but here seems a simple solution. 

Sewage issues may add some complexity, but take a look at this space.LINK

I haven’t found any listed claim over the land as it is a result of a 100 years of dredging so it’s not of any constructive value otherwise and as it is an artificial island in the Duwamish (polluted) river 

The “wetland” defense is weak.  Some of this area already looks leveled so setting up camp comes down to getting some boats. Add some rain water collection and some agriculture you have a full setup. According to this map the area is infested with Himalayan Blackberry’s