Rain gear is not required, although the place I went had some available for colder days. You will be thoroughly soaked from head to toe; you'll have mud in placed you'd never thought of. And yes, everything from your shirt to your underwear will be permanently stained from the iron in the water. Definitely wear long shorts - bathing suit bottoms won't protect your backside from the chafing of your bottom rubbing against the boat as you bump up and down. Trust me on that... saw a few red legs when we got off. If you have glasses, you must leave them at the boat house or have a tight strap to keep them on your face - the place we went to had string you could tie your glasses onto your face. We went in the highest tides, so it was pretty rough and yet everyone in our boat, even the ones who were a little scared, were laughing themselves silly.
The place I went to (different from the one in the photos), also had the best hot showers afterwards - the water was very hot and stayed hot for all 60 of us. So when everyone was shivering we were all warm very quick afterwards. Mmm... hot shower...
Oh, and beware... the mud gets into all the working buttons on your camera too, even the waterproof ones, so if yours has crevasses or visible moving parts leave it at the boat house. Honestly, you will be sloshing around so much most of the photos won't turn out anyway. (I had a lot of photos of muddy water flying everywhere... the only good photo I had was when we were heading back to the boat house.) If you have a waterproof digital camera you don't really care about, feel free to bring it. Otherwise, I recommend disposable waterproof cameras for this one, just in case it gets ruined.