One idea I've always wanted to try writing is the concept of vampires exploiting their lack of a need to breathe and generally low-maintenance biology to escape the brunt of the sun's rays by living underneath the sea, and the opportunity to use this idea in my current story has recently presented itself. So I'd like to take this chance to check if the idea, as it stands, sounds like something an actual human being with an actual human being brain would actually consider a sane idea.
Here's the background: For the past six months to a year, the human race has been granted supernatural powers on a weekly basis, and since you only get to keep half a dozen or so at a time, everyone is walking around with their own customized assortment of superpowers, along with a few permanent powers everyone got and can't get rid of. One of these, the ability to temporarily deactivate anything that uses electricity, combustion or chemical reactions just by looking at it, has helped cause the total collapse of society and infrastructure, and now humanity's living at a tech level around that of a zombie apocalypse, forced to fend for themselves to get food and supplies and stay safe.
Now, a certain kind of supernatural power that's been periodically showing up is a shapeshifting power you can't keep longer than a week, which lets you transform into a member of a new fantasy race, and if you like, when the week is up you can keep that new body and give up the old one instead. One of these races, the power for which shows up around a month after society collapses, is called the Greys, and they're functionally vampires. They don't need to drink blood (they eat normal human food instead, although they do find blood tasty) and don't have the weaknesses to garlic or crosses and the like, but otherwise they're dead ringers for them. They burn up in direct sunlight, don't need to breathe, can see perfectly in the dark, and are incredibly resistant to both high and low temperatures.
Now, somebody figures out that these traits mean they can actually survive underwater, and that when you're a Grey, going 20 feet underwater is all it takes to be completely protected from the sun's rays (it’s not the UV specifically, but a supernatural element of the sun that’s neutralized by 20 feet of water). The idea is that the severe shortages of food and the dangerous anarchy that passes for politics in the immediate aftermath of the collapse make a lot of people decide they'll have a way better chance of surviving and living in relative safety and peace if they become Greys and try to colonize the ocean from scratch, reasoning that thanks to the aforementioned anti-tech power and the total failure of the power grid, technology barely works above the water anymore either, so any society they could build down there would hardly be more primitive than up there from a technological perspective. Plus, a few powers have shown up recently that solve the whole "you can't talk underwater" thing too.
Is there any flaw in this logic that would make it clear to anyone who attempts it that even in this dire situation this is a terrible idea?