Blanch the watermelon rind by bringing 4 cups of water and 3 tablespoons of salt to a boil in a pot over high heat. Stir until the salt dissolves.
Add the watermelon rind. Reduce heat to a simmer and continue simmering until just starting to turn tender (you want them to still be a bit crispy), about 5 minutes.
Skim off any fat and drain. Transfer to a large bowl.
Make the brine by bringing the remaining 1 cup of water, 1 teaspoon salt, vinegar, sugar, pickling spice, and red pepper (if using) to a boil in saucepan. Stir until the sugar is dissolved.
Pour the brine over the watermelon rind.
Cover with a plate to keep the rind submerged and let cool to room temperature.
Transfer to glass jars, cover and refrigerate overnight before serving.
The pickles will keep in the fridge for up to 3 months.
Combine and store: Pour the hot brine over the rind. Use a small plate or a Ziplock bag partially filled with water (I use the Ziplock bag) to hold the rind under the brine. Let cool to room temperature. Transfer the rind and brine into clean glass jars (I use Mason jars), cover and chill before serving. Keep refrigerated - they'll keep several months.