I've given out thousands of wet suits as a raft guide... so take it or leave it, if it were me...
I would bring a pair of synthetic undies, either boxer briefs or maybe even a swimming speedo if you have one - something kind've tight to go under the suit - just to keep your junk out of a suit that's been worn by a few hundred people and washed by a bunch of raft guides as the last thing they do before they get to drink beer or go home
and the biggest waisted pair of board shorts you have to go over the suit (your waist will be an inch or so bigger with the suit on) - not necessary but keeps you from feeling so much like a gumby
sometimes the outfitter will have a fleece and / or splash top to go under your pfd (life jacket) - so check and see what other gear is available cause bringing an old fleece or rain coat to go over the wet suit can be more comfortable, also a pair of fleece or neoprene socks can help with the custie booties - probably the most likely place to pick up athlete's foot
good luck and don't worry too much about the gear, even if you showed up with nothing you'd be fine - you'll have a blast!
edit - btw, as far as the wetsuit versus shorts, I'd probably go with the suit - it gives you some abrasion resistance and more flotation as well as makes you prepared for the worst i.e. a front blows in and the temp drops or you swim a lot or spend a long time in the water - if it's unseasonably warm and you have some polypro shirts, tights, old fleece, etc. anything synthetic will work and you could build your own layer system that's more comfortable if you have the stuff - remember on the river it's much easier to cool off than to warm up