So I saw that kuhn rikon offers a watermelon knife.
At first I did the what-the-F shake of the head, but then I also noticed aframestokyo sells a Japanese watermelon knife. Doh.
The latter knife is $100 and at least could be used on other large veggies, but I would have to eat a boatload of watermelon before a bought a dedicated melon knife.
Does anyone have this knife?