This was my first time whitewater rafting (the website says good for experienced and beginners alike, so I figured I was fine). I'm pretty high anxiety, so I probably should have done something to warm up to this, but the other two beginners on my raft both had a blast. I was super fortunate to end up with Zach as my guide in the boat - he basically has his PhD in rafting and did a good job balancing fun and safety. The three of us in back all almost got sucked off the boat early on in a Class IV and he grabbed me and my husband and pushed us back in the raft before we could fall out. I was also fortunate to have 3 people on my raft that had done serious WWR before (Grand Canyon, Gauley) and they were in front. Everyone on our raft was young and physically fit. It allowed us to do a bunch of cool things. There were groups that had skinny, young kids and they didn't help with the paddling too much.
Things I wish I had known - you are probably going to pop into the water and swim two of the rapids, but you don't have to if you don't want to. I did the first one and it was terrifying but it also forces you to sort of feel OK about falling out. Wear either water shoes or Teva-type velcro sandals and if you have glasses or sunglasses, pick up one of the straps in the shop before you go. They have huge changing room/shower facilities so plan on bringing a full change of clothes. They have a photographer in a kayak taking pictures the whole time. The pictures are $25 each (less if you buy more) or $32 for a flash drive with a picture on it and maybe $65 for all the pictures? The pictures are expensive, but very high quality. You can request guides. I would request either Zach or Sam. You take a bus to and from the drop in and there is a latrine (no running water) before you get in the raft. You get water, pop, or beer for the ride back.