In a large saucepan with a lid, place rinds, water, and salt. Boil, then reduce heat to low boil. Cook covered for 30 minutes.
Turn heat off. Let sit, covered, for an additional 30 minutes.
Drain. Fill pot with sugar, vinegar, spices, ginger, and lemon. Some like to add spices in a cheesecloth, I like to toss them in and dig them out later. I think I get a more evenly spiced watermelon. Allow to sit for one hour, covered on stove, no heat.
Bring to boil, then lower heat. Cook covered for 20 minutes. Strain, and reserve syrup, and return syrup to pot to low boil for 10 minutes.
Separate rinds from spices, lemon, and ginger. Take rinds and fill jars a little over half way.
About the last minute of low boiling syrup, add about 5 drops of red food coloring (optional).
Ladle syrup into jars, leaving about 1/2in gap from top.
Seal with two part lids and can using the water bath process.