This is not the biblical rapture, but the undersea city from the game Bioshock. Game creator (((Ken Levine))) made fun of libertarians by having their underwater Galt's Gulch go haywire in under a generation. (((Levine))) made his other game smear nativists. No agenda, none at all kids. I cite Rapture as an idea, not of creating an underwater city as in the game, but of playing with that idea for an exit.
While Land linked to the floating island that resembles an oil platform and seemed Bond villain-esque, this is a bit more detailed. Yes, Land did add my tweets that are the genesis for this post. Buy an island, and use that island as an "anchor" to then build on, under and around for a mega-base city. Picture the island as having some non-threatening buildings there but the island would need a functioning dock for importing goods and transporting people. The island itself would act as a "Central Park" for sunshine and recreation while living would be underground on the island or extended out from the shoreline underwater. The island would be the hub of the wheel.
This sounds ludicrous; it is not feasible! Islands are for sale here, which you can see are scattered around the world but let us focus on the United States. Focus on an island of decent acreage that is not too far from the coast. How about Foster Island? That is 300 acres that is 20 minutes from the coast (zoom in center-right on this NOAA chart). Right next door is Dyer Island which is twice Foster's size but not for sale as some non-profit for teenage boys has ramshackle cabins there. Still, Dyer is possible expansion, as is Flint which is just off Dyer. Foster is a freehold island, which means you buy it outright for all eternity. Build on, up, outward and under. This is purely land for you to carve out and mold as you see fit. At 300 acres, one can house many with some density. Foster is pretty close to the mainland, so building offshore using the island as an anchor is a tougher task.
The problem with island buying is what you are allowed to build on the island. Who holds the keys to permitting construction? One would have to
This is a colony is it not? Education can be taken care of via homeschooling, the seas can feed you protein, and a few boats can provide security patrols if not automated. Foster Island provides ample space, but how many do you want in? An initial crew of 20 could form an LLC to get this going and pool resources. How many join the experiment? Be careful in screening, and it may not even crack 500. The selection process will become the evolutionary pressure for your community if it lasts multiple generations. The sci-fi series Wool might be a guide too. The Amish did not start out with many, and they effectively isolated themselves right in the American heartland. If this works, you replicate it on other islands. The Venetians had an archipelago empire.
The citizens of Rapture ended up as Big Daddies and Splicers, mutated via genetic experiments. Rapture did have flooding problems after a civil war and was in ruin. That was just a prog needing drama, wanting to smear the outgroup's ideas. One does not need to look into fiction for the storyline of Rapture. A progressive city saw many citizens abandon it after bloody riots decades ago. Baltimore has seen its remaining citizens barricade themselves inside or become parts of roving gangs dealing in exotic substances. Others have consumed genetically modified food or manufactured food, mutating into gargantuan people "big fatties" (38% overweight, 28% obese). A stunning 10% of the population have mutated so that they need another drug (insulin) in order to live. That is Baltimore.
Baltimore is a dystopia of 600,000. Your hypothetical, Rapture-like island community might be 500. What is the worst that could happen that could even compare to a Baltimorization of your current town?