What makes a watermelon seedless?
Well . . . that is one of those questions that could keep you up at night.
Thank God for Google.
~To get fruit, the seedless watermelon (called triploids) must be grown next to the regular watermelon (called diploid). The seedless watermelon is sterile and unable to pollinate by itself. Therefore, the regular watermelon assists in the pollination process for the seedless watermelon to get fruit.~ from ehow.com
I also found that watermelon is a natural diuretic and 92% water . . . That'll keep you up at night too . :-)