Watermelon soju, like its soju cocktail brother yogurt soju, is usually found on the menus of Korean drinking establishments. These two are probably my personal favorite flavors, followed by apple soju and lemon soju. The recipe for watermelon soju is dead simple - nearly 1 part watermelon, soju and sprite/ice. I only use 1 cup of soju to 2 cups of watermelon because I like mine a bit more fruity.
1 cup of ice cubes
2 cups of roughly chopped watermelon
1 cup of lemon/lime soda (7-Up, Sprite, etc)
1 cup of soju
squeeze of lemon
Put the ice cubes in a blender and grind.
Add the watermelon, soda and soju. Start out on mix then move to 'puree' mode.
Taste, add more of any ingredient to your liking. Squeeze a bit of lemon juice on top. (If you want to get creative - and this ain't the kind you get at Korean restaurants - try adding chili powder or chopped basil!) And if you really want to go over the top - serve in a scooped out watermelon half-shell like a big punch.