The best way to go is to sign on with a guide service, like Agua Azul. They even know the guys that can turn on the water at Micos in the dry season. I've always been willing to take on the adventure on my own, and there is a price for that. I've been lost on shuttles and missed days of boating, I've had two flats at once in the middle of nowhere, passed the takeouts after dark, and even had a van burn down in Texas. To me, that's part of the thrill- I'll tell you, it's not for everyone.
The water starts to get really low in January, so the ideal time to go is earlier in December or in November. I'm heading down over Thanksgiving this year, because a few of the runs take more rain to get the levels up and mo' water is always fun. That being said, the water can get too high this early. That's one of the risks.
Regardless- if some of you are planning to head down there, be good ambassadors for your sport. THe Mexican folks in this area are super nice, so don't go driving fast through the villages, get drunk and stupid in the bars, etc. I've had nothing but great experiences sown there, and besides, it ain't your country and if you forget that, that's when the horror stories come up.
Good luck- the winter boating is fantastic.