Last chance or not, it just wasn't good. In a last ditch effort, I threw it in the blender with some orange juice and a little orange peel last night. It was amazing. It tasted a lot like an orange julius, both in texture and in sweetness, though it wasn't quite as knock-you-over sweet. I liked it so much I immediately washed the blender so I could make it again before work this morning. Now I want to buy another watermelon. (My husband is screaming noooooo.)
I do have a little more - earlier in the summer we had a watermelon on the verge of going bad, so I pureed and froze it. I figured I could use the watermelon ice cubes in fruity drinks or something. My can't-waste-anything-ness can be ridiculous (who makes ice cubes out of an almost-bad watermelon instead of just throwing it away?), but I'm glad I have the promise of a few summery drinks this winter.
P.S. I don't really have any proportions on the ingredients, but I think I used about a 2:1 ratio of watermelon cubes to orange juice. If the watermelon is already pureed, it's about 1:1.