I believe http://project-entropy.com/ is supposed to be a travelling seastead. They’ve done some minor projects for fundraising but I don’t know how far they’ve progressed or if the project even still runs.
It’s an attractive project and if I don’t have my own project, I would probably join them in a few years. But I do have my own project lol If they’re still working on it, I may join then anyway for a few months just for the experience.
As for addressing the garbage patch, there is a project organized to clean it around Hawaii, but it’s not a seasteading project. My project will only clean the region nearby, it will not move around to clean the ocean.
Oh btw, welcome to Seasteading.org. Hope you’ll have fun meeting people and getting involved.