Eating watermelon seeds is not bad for a person's health. Instead, the cucurbocitrin and fiber they contain are beneficial to the body's gastrointestinal, cardiovascular and renal systems.
The cucurbocitrin in watermelon seeds helps the circulatory system by lowering blood pressure. This lowering of blood pressure helps the kidneys to function better by reducing excessive strain so the organs can efficiently clean toxins out of the bloodstream.
Especially when eaten with the watermelon's flesh, the seeds add extra fiber to a person's diet. While the flesh is a soluble source, the seeds add bulk as insoluble fiber. They help keep the digestive tract regular. The fibrous seeds also create a feeling of satiety so a person does not overeat during meals.