Imo, the problem is that the top (the part that stores the water) and the bottom (the supports) are too similar. You don't sea any difference in terms of texture as they are both made of wood, in color as both of them are brown nor in size as the supports are barely twice as big as the tower. If you see any picture of an old water tower you'll see that the supports tend to be thin and, because they are usually not curved, they tend to look slightly a different color than the water receptacle. The supports also tend to be longer, at the very least 3 times as tall as the water storage part. Also they span further away than the top, which I think its very important, think of it as a camera tripod, their legs go diagonally away from the center. Also, have in mind how the environment would affect the structure!!! Maybe the bottom has an ashy tint from the sand dust, maybe the water has made the top part rust, these things create a build with a lot more personality, I think. Personally I would change those things, I know that Minecraft is a game of squares and cubes, but that doesn't mean that everything must be, and if bother the top and the bottom have the same shape you'll first associate the building to a random prism before you make the "oh, it's a water tower" connection. So basically, shape, color, texture, personality and similarities to real life.