Has anyone seen or tried them?
There seem be different varieties.
According to this site, Cream of Saskatchewan are "very sweet and crisp and flavorful"
However the Citron watermelon is "is not your typical watermelon. The white solid flesh is inedible raw. It has been grown to make preserves and sweetmeats that are added to fruitcakes, cookies, and puddings. The best ones can be stored for up to a year. A variety that was widely grown on 1930's and 1940's"
Both those are green outside. In Costa Rica I saw some with a creamy white skin. Don't have a clue what the inside is like. Photo below.
I was prepared to forget about white watermelons until buying a normal melon in Guatemala yesterday. The guy I was with asked if I ever had a white watermelon. It seems someone grows them locally ... for export only ... otherwise I would have rushed down to give it a try myself.