I'm not sure how much it varies, but last time I was there I rode it in the early evening when it was just starting to get chilly. I was worried about being wet for the rest of the night, so I opted to wear a poncho and turns out I probably didn't need it. I thought I might've been the lucky one in our boat, but everyone else said they were mostly dry too! If you are worried at all, I'd bring a couple of cheap ponchos just in case!
Oh and while we're speaking of water rides, at KR over at AK I got SOAKED just three days later. I risked it and thought maybe they lowered the water levels on that ride too. Umm, not so much! Luckily it WAS pretty hot and we were heading home just a few hours later!