I broke my last flushe fast with Watermelon also, and it sometimes doesn't digest well for me. Consider this: odds of Watermelon coming out in approx the same size would be astronomical? Watermelon would have the consistancy of rubber fishing worm (sounds like you found the same consistancy that I did.) My undigested watermelon is about the same color as the liver flukes that I've passed.
Should have been at my house when I passed about 2 quarts of liver flukes ... took 3 days to get them all out ... WAY more watermelon than I'd eaten.