This is a nice idea for a black and white image, however, good black and white images have lots of contrast. The umbrella seems to blend into the rest of the image rather than stand out. Also, all the grass in the background is a little distracting.
Ripples in the water may have added some interest.
I think this shot would have benefited tremendously from better black and white conversion. If you would allow me to take a look at the original photo after the challenge I'd be glad to mess around with it. I find the shot very appealing except for the fact that it is very hard to differentiate between the umbrella and the background as there is not much contrast. Also, I would have composed it to get rid of that branch in the upper right and get the umbrella out of dead center. You have captured a great mood and scene that I believe can be improved upon without actually finding it again, which is always the best option.