It is well known amongst Asian medicine that Watermelon cleanses the kidneys. I usually take a vegetable peeler, peel off the outer layer of a watermelon, and throw the rest into a blender; wherr wherr wherr, and drink WITH A STRAW. So you get all the seeds down you..
If you drink enough, it WILL give you the runs, but the "good feeling type", vs something like epsom salt, which I catagorize as the "bad feeling type". Your stool will be "reasonably solid" but it is a very good flush. Also, watermelons are highly enzyomatic, and so it does clean out the colon quite well.