ShipStead
This page should be filled out with the arguments in favor of using a Cruise Ship as a Seastead condo (maybe some against too).
Arguments In Favor
Ships are known to work fairly well on the ocean. They're somewhat survivable in large storms. They're relatively mobile, and that mobility can help avoid storms, visit more ports / countries, etc. You can buy used cruise ships for reasonable amounts of money. These could then be sold off as condos at reasonable prices. This could be a lower risk way to get an ocean safe seastead going. See also Patri's ShipStead project page. Jeff Chan prefers the term "CondoStead" since a condo/timeshare is a familiar concept, easier to sell and explain, etc. Essentially this idea is like Residensea / The World, but at a lower cost and with better economic structures.
Arguments Against
Ships are not as radically different than a spar buoy, and possibly not as comfortable in large waves. Spars have much lower waterplane area than most ship types, though SWATH come close. Ships may cost more to build and operate than a series-production spar structure.