There's a trick to it. You can run a current under salt water, and calcium carbonate builds on either the anode or cathode - forgot which. Some rebel scientists have been using this to build coral reefs, since its the same thing, and the corals happily attach to it. The mainstream scientists tried to shut it down, but I think now they're trying to pretend they always did it. 

Use that to build structures. You build a wire cage in the shape you want, electrify it, and in a few days you get a shell stronger than concrete, and watertight if you do it right. This part is 100℅ my own idea. You could use this for underwater construction. The shape matters a lot, too. Barges can't go to sea because waves wreck the flat shape. So make it a vertical pile, with more underwater than above. Waves can't do much, if anything, to that shape. 

Its a lot of stuff that hasn't been done yet, so right now it would be more difficult than finding a nice cave, but I think it can be easier, once it's all figured out. And it scales almost infinitely. And it gives easy access to food.