I whitewater kayaked the Yaque del Norte last Christmas with Franz's Aventuras del Caribe, owned by Franz Lang. They have whitewater rafting just about everyday. It's a good and reputable company to deal with. They will pick you up at your hotel, serve breakfast, run the river, and lunch and a beer after for $55US (and that's gringos price direct if you don't go through the hotel). If you go through the hotel, they'll add their share of the pie.
I would highly recommend this activity, although it's not the greatest whitewater rafting you'll experience, the scenery is awesome and the drive there is interesting. There's a very high rock for cliff jumping in the river, highly recommend to do it.
Here's the info;
Franz Lang 809-574-2669
franz.lang@codetel.net.do
www.dr-canyon.com interesting pictures.
Good luck and make sure you do it.
ecarignan