(All instructions will be for 2 tiers, see note* for 3). Prepare a large cutting board or sheet pan as your work area to help catch the juices from the watermelon.
Cut off the top and bottom (it's best to give a couple inches at one end to allow you to melon ball some of the watermelon later.) Cut the large seedless watermelon in two, roughly ⅓ and ⅔ so as to get the base for two rounds.
Trim each watermelon round but removing the skin and white pith. I start with large cuts, not worrying too much about the white, and then proceed to shave off more and refine into a rounded edge. Repeat with both tiers.
Drain off as much of the water as you can. (Optional) Allow the watermelon to sit for 15-20 minutes to allow additional drainage before continuing.
Keep each round separate (don't stack yet). Use a paper towel to blot the surface of each watermelon round, especially the top edges where the chocolate "icing" drizzle will go. Set aside your sprinkles.
Melt your candy coating according to package instructions and add it to your piping bag with a ⅛" round tip. Pipe the chocolate onto the edge of the first round, squeezing periodically to make drips down the sides. Vary your pressure for more varied drips.
Quickly toss sprinkles onto the candy coating if using, before the chocolate coating hardens. Repeat for the second round, and allow to solidify. Times may vary depending on the temperature of the watermelon and the ambient temperature.
I recommend leaving the cake tiers separate for as long as possible to help with drainage. Once ready to assemble, cut or fold a few paper towels and place on your serving plate. Place the large tier down (this will help absorb some moisture as well as keep it from slipping). Then repeat with a smaller paper towel footprint between the first and second tiers.