Thanks for all the replies!
I avoid anything with artificial sweeteners. Aspartame actually gives me diarrhea; it did even before my UC diagnosis.
Regarding potassium chloride and ulceration, my understanding is that taken with food, it should be fine - keep in mind the daily recommendation for potassium is 4,700 mg. Supplements sold over the counter must contain 100mg or less, so the risk of taking too much is very low (unless you are popping half a bottle or so a day).
I'm thinking Pedialyte or coconut water probably is still the best bet for me. It hasn't been as hot anyway, so I've been feeling better the last few days. I have a GP appointment next week so I'll probably get bloodwork done just to be sure.