I did the river raft float tour, too. I'm glad I did it, but don't have to do it more than once.
They provide plenty of water for you to drink so you stay hydrated, and even though you drink a lot you really don't feel the need to have to go to the bathroom, which was a relief for all of us on the boat. It lasts something like 4 hours, so it's amazing you don't "feel the need." You end up at a marina, and when you get off the boat you will need the facilities. The lunch was fine, and you get to keep the soft cooler it comes in.
I agree with Candy...definitely wear sunscreen. There were people on our boat that didn't put any on even after we offered to let them use ours, and they had bad sunburns when they were done. Wouldn't surprise me if they didn't get sick that night from the bad sunburns.
Definitely wear a hat and bring a camera. Lots of photo opportunities.
And your feet will get wet...boat shoes would have been a good investment.
And yes, the ride in the bus to the base of the Hoover Dam is definitely an experience in itself!
If you're driving to the Hacienda yourself, give yourself plenty of time to get there. I guess there are bad backups when you get close to Boulder City because the transportation "experts" didn't realize that shoving two lanes of traffic into one lane and adding semis would cause traffic nightmares. Duh!