From Palenque, drive or catch a tour bus south a couple hours to the town of Frontera Corozal. From there, you can hire a boatman to take you up the Rio Usumacinta to the Yaxchilán ruins. The ‘lanchas’ or long narrow river boats, cost around 600 pesos ($50). You can hire your own boat or you can wait for one to fill up and split the cost. For the price, I recommend going early and not waiting on a full boat to beat the crowd to these awesome Mayan ruins.
On the way up the river that separates Mexico from Guatemala, many bird species, monkeys and the occasional crocodile can be seen.
The jungle here is very dense and full of biodiversity. The ruins are strategically located inside a large river bend that surrounds this ancient city on three sides. Many large temples can be explored here including a few large labyrinths (bring a flashlight).
Yaxchilán is unique to other Mayan sites because of its access by river only and extremely remote location.