Could always buy a decommissioned cruise ship. Big enough to be a city, plenty of tech on board, and can move around the water. Main problems would be initial cost, protection from pirates, and power generation (maybe buy a nuke reactor from Iran, too?)
Maybe get some green/renewal energy tech group onboard as well? Since the intention isn't to sail it anywhere, perhaps a towed solar panel array or some kind of current driven turbine? Or start fishing and converting them into both food and biofuel (including from that which the humans processed first)
