They go by boat from the island across the back bay and catch a river into the mainland, the river gets shallow at the village of Bomba where they transfer from the Boat to a bus, the bus is about a 40 min. ride to the next river then transfer to another boat that ends at the rainforest where the ruins are.
For me the trip there is a major part of the adventure.
Both rivers are full of wild life and the guides stop and point out things like birds , Bats , crocks, monkeys etc.
I suggest the trip to birders.