Cut top third off watermelon; trim bottom
just enough to make a flat surface. Scoop
Working in batches, puree
watermelon in a food processor. Strain
through a fine sieve; discard solids.
(You should have about 9 cups juice.)
Cover rind with plastic wrap. Refrigerate
juice and rind until chilled, about
1 hour. Combine juice with seltzer; transfer to rind.