A small tip : when you land in Punta Cana, an army of local drivers will run towards you yelling buzz-words to grab you attention : "Travelocity! Expedia!". Immediately you think that the company sent them - but, they didn't. They charge extra money for transportation.
It is best to avoid those and find yourself a decent taxi.
Once we arrived at Krabi Airport we took a taxi service to the pier (600Baht). From there we took a long tail boat that took us to Raliey beach (you can also take a bus for 150Baht per person). Since we were four we took the Taxi.
Before I paid, I tried getting outside the terminal to see if there were "free lance" taxi drivers who could give us a better price. No one approached even though I wore my "tempting" face, so I guess it is well regulated in Krabi.
The Taxi was more of a minibus, and was amazingly convenient. We arrived at the Pier after approximately 45 minutes .
At noon we left for San Cristóbal. The drive took us 5 hours. You can use ADO but their hours aren't comfortable (only early in the morning or late in the evening, in which you reach San Cristóbal in the middle of the night). We used a local company called AXA which has buses to San Cristobal at 13:30 and 17:00. 13:00 was great as we arrived at 18:30. Another option if you don't want to sleep in Palenque area, is to get to Palenque in the early morning, take the bus that leaves at 7:30 which gives you a tour at 3 locations (archaeological site, Agua Azul and Misol Ha falls) and brings you directly to San Cristóbal. I don't recommend that because you will miss the El Panchán experience, and won't have enough time to get lost in the jungle.
There's a tour to the remains and the falls leaving from San Cristóbal. Don't do it! The drive alone is 5 hours in each direction, so you're left for two hours to do all of the sights all together. Definitely not enough.