Cut your watermelon into a cube, transfer it onto a baking sheet and let it freeze in the freezer until the watermelon is solid, about 2 hours.
Transfer the watermelon cubes to a blender or food processor and blend until smooth.
Divide the blend among two loaf pans (or put it all in one deep baking dish), packing it down as you add more on top.
Transfer the pans to the freezer. Freeze until the sorbet is scoopable, 1 to 2 hours more. To serve, scoop the sorbet into dishes and eat immediately.