Well, there is some seasonal work, but it's not the norm and it's probably difficult for most people to live working only half the year.

Seasonal workers tend to migrate where the work is so most of them do work year round just not in the same place or even field.

You also need to differentiate between just normal rig workers and deck hands and highly skilled professionals e.g. underwater welders.

I work in Boston and I see people working in the winter in buildings. What region is this?

Depends on the type of construction specialty. I have a neighbor who is in heavy construction (i.e. drives backhoes, etc.) and he's typically off for the winter--though he sometimes does various other things.