The only technical porblem with your photo is the centering. I personally have no problem with centered photos, this one, however, is just a hair off-center. But not everyone will notice that, so that problably isn't why it didn't score better.
I think the reason it didn't score as well as it could have is that the bubble's look fake. Whether they are fake or not is irrelevant. The image itself looks like something you constructed in photoshop, or printed and re-photographed. Either way the voter's here saw that and voted accordingly. One thing you'll discover about the DPC crowd is their eye for seeing 'illusions' or whatever you want to call them. If it looks fake they will vote accordingly and that's all there is to it.
If this image was a pure photograph with no editing or illusions, than all I can say is you did your job too well! Not much more I can say. :)
Cool. I like the bubbles. I hope you post how you did this after the challenge is over. For me, I prefer off centered objects as they add more interest to me, but overall - well done. 7