Watermelon leaving drinks

So, mom left. We had a small leaving party for her, and she was very happy to meet our friends and felt good about her month in Berlin. I made watermelon cocktails to serve before we all move downstairs for dinner, but they were not as good as they sound. I don't know why, but after pureeing and sieving four kilos of watermelon, the result was only decent. I added some lemon juice, but there was still a vague, bitter aftertaste to the juice. We have now started our weekend with a big glass of watermelon juice every morning, and I'm warming up to it.