Place watermelon in a blender and blend until smooth. Pour through a fine-meshed sieve set over a bowl. Discard solids.
Measure out 4 cups of watermelon and pour into blender. Add sugar, lime juice, and salt. Blend until sugar dissolves, about 1 minute.
Pour watermelon mixture into a large bowl. Add half-and-half and whisk to combine. Refrigerate for 1-2 hours.
Freeze mixture in an ice cream maker according to manufacturer's instructions.
Meanwhile, make the crust. In a bowl, stir together waffle cone crumbs, melted butter, and sugar. Press mixture into the bottom of a greased 13x9-inch freezer-safe baking dish. If you want, line the baking dish with parchment paper to make it easier to remove the bars.
Spoon sherbet over crust smoothing with an offset spatula. Spread whipped topping evenly over sherbet.
Freeze overnight or up to 3 weeks.
Cut into squares and serve.