Cut the watermelon into slices and remove rinds. Chop the fruit into chunks. If your watermelon has seeds, there's no need to remove them, unless you prefer it. Place chopped watermelon in a freezer storage bag and wait at least two hours for the watermelon to freeze.
When the watermelon is frozen, remove it from the freezer and let sit on the counter for about 10 minutes, until they soften a bit.
Add the lime juice, water, stevia, raspberries, and mint leaves to a high-speed blender. Pulse that for a few seconds.
Add the softened but still frozen watermelon to the blender. The watermelon will break up into pieces. Continue blending until it transforms into a thick, slushie-like consistency. Taste and add additional stevia if necessary. If the freezer locks up, it is a sign the watermelon is still too frozen. Let it sit for a few minutes and then blend again.
Pour the slushie mixture into 2 glasses. Garnish with watermelon wedges. Serve immediately.
You can refreeze any leftover slushies. Remove from the freezer and allow it to sit at room temperature until it softens to a slushie consistency.