Line a baking sheet with parchment paper or a silicone mat. Set aside.
Place white chocolate in a microwave-safe bowl. Microwave in 20-second intervals at 50% power, stirring thoroughly between each burst, until chocolate is melted and smooth with no lumps. This should take 1-2 minutes total. Be careful not to overheat.
Pour the melted white chocolate onto the prepared baking sheet. Use an offset spatula to spread it into an even layer about 1/4-inch thick, forming a rough rectangle approximately 9x12 inches.
Working quickly, place the dried watermelon cubes onto the chocolate layer, pressing them gently so they stick but don't sink completely. Freeze the watermelon pieces for 10 minutes beforehand if you have time.
Sprinkle the chopped mint evenly over the surface, then add the flaky sea salt. If using, dust the freeze-dried watermelon powder over the top for extra color.
Transfer the baking sheet to the freezer and let the bark set for at least 20 minutes until completely firm.
Remove from freezer and use your hands to break the bark into irregular shards. Store immediately in an airtight container in the freezer until ready to serve.