Remove thin, outermost green skin from watermelon rind, and cube rinds.
Place cubed watermelon rinds into a bowl, sprinkle with sea salt, toss gently, and allow to sit for 30 minutes. Toss again gently and allow to sit for another 30 minutes (total salting time = 60 minutes)
Make Kimchi Seasoning
While the watermelon rind is salting, make the Kimchi Seasoning by combining watermelon puree (if using), garlic, ginger, gochugaru, rice vinegar, and fish sauce in small bowl.
Make Watermelon Rind Kimchi
Drain salty water from watermelon rinds, rinse a few times with fresh water, drain, squeeze out as much water as you can with your hands, then set aside.
Pour the Kimchi Seasoning over the watermelon rinds then stir everything to combine. Make sure the rinds are evenly and well coated; using your hands to gently massage the seasoning into the rinds is highly encouraged, but wear gloves!
To eat right away, garnish with sliced scallions and toasted sesame seeds. Within the first day, the Watermelon Rind Kimchi will taste like a highly seasoned, spicy salad.
Place the Watermelon Rind Kimchi into large glass jars, pressing down to remove air pockets. Cover with a tightly fitting lid and store in the refrigerator up to 3 days.