Since there's global warming everywhere. If so, Boston (being a coastal city) would be underwater, no?

Global warming everywhere? Cite please.

http://www.gamesas.com/topic/1591118-this-is-boston-right-where-the-hell-is-all-the-snow/

Global warming is not the cause. In new vegas we see snow. The reason we dont in fallout 4 is the developers did not include changing seasons, nothing more.

Well the lowest elevation in Boston is directly at sea level. In comparison to say a place like New Orleans, where the average elevation is below sea level, Boston wouldn't be in nearly as bad as shape as a place like NO, which probably wouldn't even exist in the FO Universe anymore. It seems like there are some areas that are overrun with water, so I guess it is alright, given that Boston also doesn't have the reputation of being hit by hurricanes and what not, like New Orleans does. Really, the places that probably shouldn't exist in the FO world would be New Orleans and say Miami, places which are high risk in our world and not even taking into account nuclear war.

Yes the sea level has risen, but I doubt it is in any realistic or consistent measure.

As Evl pointed out, New Orleans SHOULD be underwater, but Obsidian's ideas of a Fallout game set there would only mean that wouldn't be true.

The water level has only risen in places, and to such levels, that the developers find it makes for a more interesting game world.

We see snow in the mountaintops. That doesn't reflect the temperature at lower altitudes.

In fallout 4 Cait mentions how hot it is when it's sunny, despite it being autumn.