2. Tourism
That's right, tourism! People may want to visit an underwater city and by making it accessible to the public; well, let's just say it'll be like the 'Las Vegas' of the ocean. People from all corners of the world will want to experience the vast unknown that is the ocean. Heck, throw a couple of buffed guys in a 'Big Daddy' suit and make them walk around the ocean floor. Rapture might be able to financially sustain itself based on tourism alone. Add some fishing and sea salt exports and you've got enough money to expand the city!