According to Jain versions of the Ramayan, Shri Ram crosses the river Ganga with the help of the Nishad (tribal) chief / boatman, commonly known as Guha (also called Kevat in many retellings).

So, in simple words: A Nishad boatman‑king named Guha (Kevat) helps Shri Ram, Sita and Lakshman cross the river Ganga.

If you want to see how Jain Ramayan differs from the usual (Hindu) Ramayan, you can read more here