If chafing is the concern, the ponchos are probably NOT going to be effective. While they will keep most of your hair and also your upper body dry, the water sometimes completely fills the boat for short periods during and right after the bigger drops, and *WILL* get under a poncho. I know this because I tried it with just that goal in mind, and it didn't do me any good at all; I still got wet britches and ended up miserable for the next 3 days.
We have a similar ride at our local 6 Flags, and I've experimented with how to prevent the chafing problem. Other than wearing a 2 pc plastic rainsuit with a tight drawstring waistband on the trousers(which is effective, but too hot in summer), the only way to be *sure* to dodge chafing is to change clothes.
I bought a pr. of lined ripstop shorts and a bikini top, then put a mesh t-shirt over it. I change into this before riding Kali or any ride where I'm liable to get very wet, then change out of it as soon as I've finished riding. I keep a large ziploc in my backpack to put the wet clothing in, and bring a swimmer's towel to use to dry off. Shoes are not a problem because we always wear sport sandals in the parks.
I don't trust the center well to keep our things dry. I just pack a large trash bag along, and we put our whole backpack and camera bag, as well as all wallets and watches, inside of it and knot it up before riding.
BTW, here's irony for you: DS insists on wearing a poncho on Kali, b/c he doesn't like getting hit with big waves of cold water. However, as soon as we finish riding, he invariably takes off the poncho, then heads for the nearest splash fountain with the goal of getting completely soaked!