Once someone asked Ramkrishna Paramhans if a Guru was necessary on the path of Sadhna, on the journey of liberation ? Just then a steamer ship was passing by on the Ganga with a boat tied to it. Ramkrishna told his disciple that the guru is like that steamer which is pulling the boat along. The boat which you can imagine as the disciple is also moving at almost the same speed even though there is little effort on its own part. One may ask that is the boat not capable of crossing the river by itself? The answer is Yes it is, of course. But the time it will take to cross will be very long. Not to forget the tremendous effort the boatman will have to put to accomplish the goal.
The very first shloka of Guru Paduka Strotam by Adi Shankracharya also says that the Guru is like a boat which helps us to cross the river of Anant Samsara.
अनंत संसार समुद्र तार नौकायिताभ्यां गुरुभक्तिदाभ्यां।