I wanted to suggest a book I bought. It is Good Housekeeping Stain Rescue. The A-Z guide to Removing Smudges, Spots & Other Spills. I ordered it through a book club but I am sure you could find it in stores or on Amazon. I looked up watermelon, although it did not specify watermelon, I looked up fruit, and it gave the directions as follows (although, you may have tried this already)
1. Scrape off exxces fruit
2. Flush under cold running water
3. Launder, using the warmest water and the type of bleach that are safe for the fabric.
In each section, it gives directions for FABRIC, UPHOLSTREY, CARPET.....in the upholstrey and carpet section, it suggests making a mixture of one tablespoon of liquid hand dishwashing detergent with two cups of cold water and sponge the stain with solution.....on carpet it mentions the same mixture however adding household ammonia (staying away from the sudsy or scented ones)It says ammonia works well on dried blood, perspiration, citrus, juice, flet-tip pen, urine, and acid based stains. Hope this helps, THIS BOOK IS FULL OF KNOWLEDGE!