Just as I have thought the rind of a watermelon is fantastic to get pickled because it is edible and it gives that crunch and taste for a perfect side dish when you are eating or just snack on it if you have a craving.
The process is easy and straightforward and the video shows how to do it but I think you could also shred it or cut it in any way form or shape you wanted and could even put some natural flavoring and herbs in it too.
Source
The recipe calls for and equal parts of water, sugar, and apple cider vinegar of four cups each for a small sized water melon shown in the video, you have to put some two tablespoons of salt, a tablespoon of whole black pepper or less if you prefer and a teaspoon of dried and flaked hot chili peppers.
You boil it and simmer it for a few minutes and then cool it completely before putting it in the clean jars where your watermelon rind packed in already. Seal the jars and turn them upside down before placing them to the refrigerator. After two to four days they are ready to eat. No need to ferment them though because the pickling liquid is sour already although without beneficial microbes. After two to four days we can enjoy our pickles.