This was just right for me. It was sweet enough which must mean it depends on the watermelon being used. Mine was fully ripe. I didn't blend the melon, just drained off juice from a big bowl of watermelon cubes through a mesh strainer in a measuring cup

I give this 4 stars with my improvements - It isn't really a Cosmo with only vodka in it. So I added in 3/4 oz of Triple Sec and a whole ounce of lime juice. Also, because it's February and all I could get were two little baby watermelons that weren't that ripe I threw in a generous pinch of Splenda. If it were Summertime and I could find some nice ripe melons I'm sure the sweetener wouldn't be necessary at all.

I had some mushy watermelon so this seemed perfect but it was extremely bitter. This recipe could really use a sweetener.