You will know if it goes bad. Lol I've tried watermelon twice. The first time was a mess and it got tossed out before it cleared. The 2nd time I fermented it in my garage during the winter. Ambient temp was about 50F. It took quite a while and there is still a little funk when you smell it, but it tastes pretty good. This is what I started with both times.
4-gal bags of frozen watermelon.
1.5t pectin
1t tannin
1.5t yeast energizer
1t yeast nutrient
1.5 lb sugar
1g french oak chips
yeast starter
sg of juice 1.04
sg after sugar 1.073
After it cleared I added 1gal of watermelon juice, acid, and more sugar. I can't find my notes at the moment for quantities.
If I did it again. I would try to remove as much pulp before fermenting as I can, and add the acid at the beginning. It went through a weird slimey stage during fermentation. It went away though. I think the cold helped. A 1gal batch might be better.
I'm guessing you will need more sugar.