We wore ponchos, and as long as they are full length (so you can sit on them), you will keep pretty dry.
The sides of the rafts are open so water splashes in the whole time - it's not like you get inundated, just thast water splashes in all the time.
One ride we did everyone kept dry until the really calm bit after the big drop, and the water just hit at the right angle, came in, and absolutely soaked the couple next to that opening.
It's a great ride and a lot of fun, but unless you enjoy walking around soaking wet, wear a poncho. If you do wear a poncho you'll still get a little wet, but not enough to bother you.