Watermelon seeds, contain citrulline, a non-essential amino acid that converts into the amino acid L-arginine when eaten. Citrulline acts as an antioxidant, as well as a vasodilator, according to the USDA.
Laboratory experiments on animals, proved that Watermelon Seed Oil (Citrullus lanatus) was able to reduce the size of the prostate, in only a few weeks.
The experiments used MECLS (Methyl Extract) of Watermelon Seed Oil, but a far purer version is the Cold Pressed Watermelon Seed Oil, which contains none of the toxic methyl.
“In addition, watermelon has been widely used as a medicinal plant in African and Asian cultures due to the presence of several compounds with phytochemical activities . Watermelon seeds have strong antioxidant activity [3,4], and due to their diuretic and purgative activity, have also been used in the treatment of gastrointestinal diseases , urinary infections , gonorrhea, leukorrhea  and prostatic hyperplasia . Watermelon seed oils are classified as high quality oils due to the presence of x-3 and x-6 fatty acids  and phenolic compounds such as gallic, protocatechuic, p-hydroxybenzoic, vanillic, caffeic, syringic, p-coumaric and ferulic acids  .”
Furthermore, I choose the Cold Pressed Watermelon Seed Oil over the Methanol-extracted Watermelon Seed Oil, as it is chemical free, and yields far purer compounds.
Many scholarly articles discuss these issues:
Scientific studies on watermelon extract.
A variety of peer reviewed studies on watermelon extract and prostate size.