I did this about 2 years ago and it was one of the highlights of my cruise. I booked thru the ship. The name of the company was Rios Tropicales
It was on the Reventazon and was class II and III rapids....perfect for a first-timer....and was about 9 miles long.
They provided a great lunch. They also have a very clean and modern changing room with showers.
I have some pics posted, if you want to nose around: