One of the recent talks regards expanding our cities to colonize the oceans, which many find to be more feasible than colonizing another body in space. So far, we have not gone as building a permanent, metropolitan establishment, but when that comes, some issues obviously need to be addressed.
Fishing and greenhouse farming have been considered, so finding food is not a problem.
Other problems are not so straightforward to solve. In the maximum depth of 350 feet, pressure is added, perhaps close to human tolerance, so what our main facilities need is an architectural shape best suited to withstand that kind of pressure. By choosing the three basic shapes--pyramid, box and dome--which one suits maximum?
In relation, glass does not react well to added pressure, either. That's why deep-sea diving submersibles have thick windows that are small in area and/or circumference. At a maximum depth of 350 feet, would a window in a permanent underwater city also need to be small and thick, too? If so, by how much?
Certain materials withstand pressure differently. Roman concrete has this unique ability to harden as it gets older and keep water out, which may explain how it has survived the past 2,000 years. Would this work in a marine city with a maximum depth of 350 feet, or would something else make for a more durable foundation?
Another problem is how to get a constant supply of oxygen. I could personally suggest planting each main facility with a chimney tall enough to break the surface, but is that feasible?
If one wants to spend the rest of his life beneath the surface, he would prefer to be dry. So how does one open and shut doors without drowning the entire corridor in the process?
The one final issue is finding fresh, drinkable water. I have no idea how that could be accomplished.
How would all those listed issues be addressed and solved?