After removing pink flesh, and using a vegetable peeler to remove tough green skin, cube watermelon rinds.
Combine vinegar, water, sugar, sea salt, and pickling spice in a large saucepan. Bring to a boil.
Boil watermelon rinds for 2 minutes before reducing heat to a simmer and cooking uncovered for 10 minutes.
Set mixture aside for 1 hour to cool. Pour mixture and rinds (they should be translucent) into glass, airtight jars and store in refrigerator for up to 2 weeks.