According to a news story:

"Watermelons contain an ingredient called citrulline that can trigger production of a compound that helps relax the body's blood vessels, similar to what happens when a man takes Viagra, said scientists in Texas, one of the nation's top producers of the seedless variety."

So come on over and let's conduct our own experiments.  Also let's make sure it is served at the next M & G.  

They said the one down side was that it is most consentrated in the rind. Im UP for trying though....