/sci/ - Science & Math

There are some areas in the middle of the Indian ocean just shallow enough that you could probably use concrete pylons to build on top of. You'd definitely deal with pirates though.

With electricity and magnesium cages, you can grow a strong cement-like structure from seawater. Sort of like a hardened limestone.

Read The Millennial Project, specifically the bits about Aquarius. He laid out a very extensive plan to create self sufficient floating cities. A few mistakes in his seacrete formula and the use of OTECs to power it though. Plus the start-up costs would be in the billions.