Hollow out melon: Scoop out watermelon until only pale green interior shows; refrigerate shell.
Make punch: Puree 4 cups watermelon pulp in batches in blender until smooth and transfer to large bowl. Stir lemonade and limeade concentrates into bowl with melon puree until completely melted. (Mixture can be covered with plastic wrap and refrigerated overnight).
Add fizz and garnish: When ready to serve, gently stir seltzer into punch. Pour punch into watermelon shell (refrigerating any excess) and float lemon or lime slices on top.
Serve with sherbet: Ladle punch into glasses, then float small scoop of sherbet on top of each glass. Serve.