The rule of thirds goes like this: divide your frame into three equal areas, by “drawing” two lines both horizontally and vertically (two lines each—think of a tic-tac-toe board). The subject should be placed at at least one of the intersections of these four lines. A human form, for instance, would normally be placed along the left or right 1/3 of the frame, a horizon at the top or bottom 1/3. Avoid centering your subject unless you have a valid reason to do so.
As far as this shot goes, I’d have to agree with Jeruba that this shot is merely documentation. A snapshot. Aside from not fitting the rule of thirds (lots of empty space up in the sky, not really serving any purpose), the subject is kind of bland. It doesn’t really say anything. Well, it says something, but it doesn’t really speak anything. The sentiment in the graffiti is a cliche, and thus, so is your photo.
As a general rule, anything shot with a normal focal length lens from eye level is going to be a boring shot. It’s the same perspective we would see if we were standing on that beach. You have to show us something we wouldn’t see if we were standing there. Something to contrast or reinforce the message in the graffiti. Different angles. Play with depth of field. Get people in your shot.
But, as others have said, great clarity and color. The empty space in the sky might not necessarily be bad if the shot was to be used in a magazine or something similar, where there needs to be text.
Looks like you’re on the right track though, looking at some of your other photos (I especially like Fire Escape and Back Door Open). Keep shooting and seeking feedback.
I hope this critique is taken with as many grains of salt as needed. I have been in photography for a while, and, while I was a professional by definition, I am by no means an authority. I’m just giving my honest assesment.