Why make it complicated?
I (well, not I per say) cut off the rind from a watermelon, removed the seeds, and froze the lot in bite size hunks.
Lesser folks might thaw and eat straight, but I am enjoying putting the pieces into a blender and drinking as a smoothie (pure watermelon juice and nothing else).
More surprisingly, it tastes just as good (well, different, like wholesome water, really) with maybe twice as much water added.
Ginger, salt, and lime have also been added (in varying quantities and to great effect), as there's a lot of fruit in a watermelon and plenty of opportunity to experiment.
If they are still in season, when this one is gone, I think another may well be in order. But for now... um, let's just say, there's plenty in reserve.
As awesome as these were, I'm pretty sure it was hunks of frozen watermelon that blew out the blender. I mean, the blender did not hold up to a single watermelon eaten this way. Of course, it's the second blender in the warranty cycle. So, either we are doing something wrong (grinding gravel and/or frozen watermelon, for instance) or the particular blender in question simply sucks.