I'd listen to your body, if you suspect you have issues with watermelon then do not eat it. At least not now while flaring as we all have more food tolerance issues while flaring.
There is no right-or-wrong, universal, one-size-fits all diet that helps all of us with UC. Often it must be individualized personto-person based on his/her responses. Keep a food and symptom journal. Write down what you ate, your UC symptoms the next day, and see if you can find any correlations (i.e., every time I eat spicy chili I have a bad morning) on multiple occasions. We can have random bad days, so you want to make sure that there is a repeat, that food x consistently bothers you on more than one occasion. If you find such foods than avoid them.
Some try elimination-diets. You essentially start with a baseline, eat the same breakfast, lunch and supper for a few weeks. You then gradually add other foods back in and look for undesirable responses.
It's going to take time and patience to tailor it to what affects you.
I tried this process of trying different diets and eliminations and found nothing helped, so food intolerances aren't always a thing. For some of us UC is just totally random and awful.
Diets are kind of the wild-wild-west, with not a lot of research and evidence. SCD, IBD-AID and others are often mentioned and things you could try. Here's a good read on what research?evidence is available for many of them:
/www.dietvsdisease.org/ulcerative-colitis-diet/