Preheat the oven to 400 degrees F. Spread the coconut on a baking sheet and bake, stirring occasionally, until golden brown, 5 to 8 minutes. Transfer to a bowl and let cool.
Combine the condensed milk, heavy cream, lime juice and a pinch of salt in a small bowl. Put the watermelon in a large bowl and drizzle with the condensed milk mixture. Sprinkle with the toasted coconut and the cinnamon.