In the mall map (the indoor map from the basic marvel starter), in a tournament game would the fountain be considered normal hindering terrain, or water terrain (which would not affect ranged combat)?

I think it's safe to say that it's "normal hindering," but that would only be my "common sense," not any actual hard knowledge from a judge or anything.

Typically it is considered normal hindering terrain since there isn't really a full square of just water. However, ultimately it is up to your judge and how he wants to rule it.

I'd call it water terrain.

However, I believe that the rest of the fountain is all blocking terrain, as it's surrounded by a thick black line. Since the map is indoors, that makes it impassible, right?

There is no thick black line around the fountain. It is hindering terrain, and you cannot specify the parts of a square that are water and another part that is fountain. It is considered hindering terrain.

Now... hindering terrain is denoted by a thin line that follows the contours of the object (fountain, diving board, bush, etc). Even if it looks thick (like some of the trucks), since it follows the contours of the object, it is hindering terrain.

If it is a thick line and it follows the edges of the squares, then it is blocking terrain (buildings, walls, EBT, etc.). The Fountain is not blocking terrain, but the receptionist's desk on the Museum map IS blocking terrain.

Wayback there was a thread about this. There are two different maps with fountains. One has a thin line and one has a thick line. One is hindering, and the other is blocking. The guy before Gandalf made a ruling on this, identifying particular squares on particular maps. It was quite controversial at the time.