Iguana Mama is good, but on the north coast. If you make it up here check it out. Why are you looking to stay in the south? Kind of curious.
There is "whitewater" here, but not sure of the quality of any of the rivers as I have not run them myself. Being a kayaker, I often hear from non paddlers how "good" a river was only to be disappointed when I got there

I recommend checking out Rancho Baiguate. We have been there, but never stayed a night or took their excursions. Looks good, though, you can check them out at
http://www.ranchobaiguate.com.do/english.htm
I can only say that the Jarabacoa/Constanza regions is one of the most beautiful areas I have ever seen. Close call with the Icefields parkway (Banff-Jasper drive in Alberta, CAN) and Samana. It would be pretty cool to see on a mountain bike too, but I wouldn't have the lungs nor the legs to even try it!
Good luck, and safe biking.
marc