Nuclear war escalates throughout the world, quickly enough to kill much of the world population, but slowly enough to allow groups to find shelter (say a period of a month). The fallout leaves much of the world uninhabitable and as a result, two main groups arise: those living far underground, and those living at sea.
There already exists a number of underground nuclear shelters, ranging from small backyard shelters to excavated mountains occupied by the US military. This group, however, would be far larger, almost city sized, and occupy a similar amount of land. They would be able to access the outside world for very brief periods, but would not be able to travel far. All food would have to be created inside, water would have to be purified and come from local sources, and energy would have to be at least partially renewable (looking at a period of a year or two, with thousands of people). Underground cities have existed, and still exist all over the world, but this one would be different given its isolation and need for sustainability. Previously existing supplies of fuel, water, and food may exist, but would eventually run out. The ocean bearing survivors would consist of a convey of boats and large ships (potentially cargo or cruise ships) fastened together to some degree in the Pacific Ocean. It would be far enough out to avoid major fallout and it would be able to move locations to avoid storms or find better food sources. People could come and go, but most would stay with the fleet for the protection and resources that it brings. This group could also incorporate things like submarines or oil platforms.
Major issues would be related to resources such as food, water, energy, and ability to remain safe from the fallout. Considering these two situations and the challenges they face, which group would have a better chance of surviving? Which group could maintain a better quality of life?