With the rise of global warming and our leaders not seeming to do enough about it, expanding our ranges past the earth's ground seems like something we must do eventually. Everyone always points to the sky, but the earth has much more room than what is just dry land. In this scenario, we instead build to the seas. The easiest way would be to build around the cities we currently have to keep the water out. My question is how would you do this? If the water becomes too high large walls might not be enough. Are there any new nanotechnologies that could build a dome that would allow sunlight through but also strong enough to keep the water out?
Simple quick basic answer is we already have that tech now.
Note other architectural suppliers are available https://www.britesolar.com/ It is relatively easy to build transparent solar glazed domes that can have liquid glass to shield from marine environments.
For an example of an underwater restaurant see https://en.wikipedia.org/wiki/Underwater_habitat#Ithaa_undersea_restaurant
By Alexey Potov - Noblige - Taken by Alexey Potov - Noblige, CC BY-SA 2.5
And for villas see https://edition.cnn.com/style/article/floating-underwater-villas/index.html
If you wish you can book your underwater stay at https://www.booking.com/articles/5-unforgettable-underwater-hotels.html or place a Poseidon resorts reservation :-) at http://www.poseidonresorts.com/overview.html#sliderReserve
The main question has to be why since historically when water encroaches we naturally move to higher ground. One scenario is that the cost of higher ground becomes more than the cost of spreading into the sea. thus it is highly likely in the future. Currently the value of protecting a costal city is unlikely to bear the prohibitive costs.
Most historic and present applications are based on research or tourism where the purse may be available for esoteric projects.
What form could it evolve through? It is possible in a country rich enough to allow individuals to build/rent personal pods that a coastal village with interconnecting links/hypertubes could easily be compartmentalised in the event of local failure but the need for redundancy would suggest an outer segmented transport hub rather than the star shaped schemes often proposed.
Since I'm assuming said dome is going to be pressurized with breathable atmosphere, the key factor is depth. For every foot of salt water (fsw) you are adding .445 psi of pressure per square inch of surface area. That equates to +1 standard atmosphere (14 psi) every 33 feet of depth. At shallow depths, say < 100 feet this should not be much of a problem. Safety glass might could be an option (albiet a very heavy one) or even something lighter like acrylic. You could also pretty easily snorkel in oxygen, power cables, communication cables, etc.. from the surface at that depth, possibly even have some service tunnels for people and supplies - even evacuate if needed.
But at greater depths once you leave the area of the continental shelves and are in open ocean (say 300 meters though there are certainly much deeper areas) you are under tremendous pressure, approaching the hull crush depth of most Submarines - and those are solid steel monohulls with no windows. You can buy a little more depth if you increase the pressure of your atmosphere but people have to acclimate to it at a controlled rate. The chances of safely evacuating from that depth are remote. There is little light and little life, it would be a very challenging place to survive. I've operated robots at that depth (and deeper) but I would be very worried to personally be in anything man-made at that depth.
But building it is only the first challenge - subsea engineering is hard. Your life relies on how well your machinery is working. If there's an oxygen-plant in the dome, oxygen is an accelerant - a tremendous fire hazard. Liquid oxygen is compressed at a ratio of 742:1, even if there's no ignition source the force of even 10 liters of LOX expanding to room temperature would probably be enough to damage that dome. For a crew of say 100 you're probably looking at more like a 1,000 liter LOX plant. If that blew it would be more than enough to shatter the whole dome. Best to keep it like a mile away from the dome and connected by hoses - and if there's a problem then send a robot to fix it.
Electrical power is another concern - you can't lose it. Period. If there's an electrical fire and it disables the power distribution, now you're down to battery backup power and emergency lights - that may be game over if you were trying to bail out a leak then you can't power your bilge pumps anymore. The water wants to get in, the machinery breaks down, it would be a tough way to live and you would need support from the surface. Put everyone in one big dome and a single point of failure can kill everyone. But have a lot of domes connected with service tunnels and at least you can seal off an area with watertight doors if you're in an emergency.
Hope that helps and I'm happy to share more if you want