Sorry Nick, I just saw this post.
Any terrain with a little blue color is a swamp the rest are marsh with the exception of the four lake hexes on map 80 and all the ocean hexes. On map 81 those are marsh hexes.
If you look on page 3 of the Saipan scenario book, it will explain the difference in the two terrain types, as it mostly about different movement costs and what type of units can navigate it or not.
Also, here is a terrain chart Guy made that I recommend you print off, as it is help:
http://www.boardgamegeek.com/image/15220...aipan-1944