Cut the water melon in half, then cut each piece into quarters. Carefully remove all pink flesh, then using a sharp vegetables peeler, completely remove all of the green skin to reveal the rind. Cut the rind into equal sized pieces to ensure even cooking, I liked 1 inch rectangles. Reserve the pink flesh for eating or making sorbet, you should end up with about 3-4 lbs of rind, depending on the size of your watermelon.
Lightly crush the Zanthoxylum berries to release their scent then bruise the leaves lightly as well for the same effect.
Next bring the water, vinegar, Zanthoxylum berries and leaves, lime zest, watermelon rind, salt and sugar to a simmer in a saucepot. cook the mixture for 30 minutes or so at a slow simmer until the rind is tender. Discard the leaves and lime zest at this point, or add it to the jars as a garnish. The watermelon rind should be completely tender, and the juice should be thick and syrupy, if the juice isn't thick, remove the rind and reduce the syrup a bit more.
Pack the rind into sterilized jars then top with the hot syrup, making sure to leave 1/4 inch of head-space at the top of each jar to discourage explosions.
Divide the Zanthoxylum berries evenly between the jars, seal the jars then process for 15 minutes in a water bath canner. Remove the jars from the water bath and allow them to cool. When the jars are cool, inspect any for unsealed lids. If some of the lids didn't seal, you can repeat the canning process, or just keep them in the refrigerator.
With the jars that sealed correctly, label and date them, then store in a dark place for a few months, or as long as you can wait to enjoy them.
I was trying to capture the essence of the Zanthoxylum aroma here, you can, of course add other seasonings. Things that go good with lime would be great: cardamom, ginger, hot chillies..you get the idea. It would be great to try making this with brown sugar, honey or maple syrup too as the sweetener.Remember that different species of zanthoxylum will have more aroma than others. If you know an area where they grow, inspect the leaves of each individual shrub. I crush and smell them side by side to find those with the strongest aroma.