Perhaps more experienced people can support/refute this -
I tried making a Strawberry/Watermelon wine that failed. I was later told that watermelon very quickly begins to rot once cut open, and that the developing bacteria tainted my must.
They said I should have started the strawberries, and once the must was fermenting at a good pace, cut and add the watermelon so it wouldn't have time to begin rotting.
How is your batch coming along?