Actually, I think the moon is an easier environment than a deep sea colony. The pressure difference between inside and out is only 15 PSI. Under water, it's 15 PSI every 30 feet, and you definitely don't want to build an undersea colony in the first 30 feet because of waves and ships. An undersea colony wold also be very easy for terrorists to sabotage. A lunar colony would be much harder.
BTW, much of California is hundreds of feet above current sea level. If you raised sea level a couple of hundred feet, we would have an inland sea in the central valley, but only around Sacramento. Cities to the South are too high to be affected. LA would have another inland sea, but only a small one: The official elevation of LA is 285 feet. San Diego would have an inundated Mission Valley, but much of the non-downtown part of the city is up on mesas.
I doubt we would become more desert because of the inland seas. The California deserts are all east of the mountain ranges and are very lightly populated. They might become even hotter in the summer. They already reach 120 F some days.
Raise sea level 200 feet and Florida is almost all gone.
