Most of what you say is true, and Watermelon is an old natural rememdy for kidney cleansing and even dealing with small stones.
If this was all that was said there would be little or no reason to even comment on the post. However M/L went way out in left field with his "scientific" explanation that had nothing to do with the natural osmosis through the cellular membrane to keep a proper mineral and fluid balance inside the cells. Remember that urination helps rid the body of excess minerals as well as metabolic waste, excess fluid and natural and unnatural toxins.
As a Southerner I love salted Watermelon and don't care if it makes me "go" more than usual or not. Its a wonderful treat and any health benefits is just icing on the cake so to speak.