"This human form of body is a most valuable boat, and the spiritual master is the captain, guru-karṇadhāram, to guide the boat in plying across the ocean of nescience. The instruction of Kṛṣṇa is a favorable breeze. One must use all these facilities to cross over the ocean of nescience. Since the spiritual master is the captain, one must serve the spiritual master very sincerely so that by his mercy one will be able to get the mercy of the Supreme Lord.
A significant word here is acyuta-balaḥ. The spiritual master is certainly very merciful to his disciples, and consequently by satisfying him a devotee gets strength from the Supreme Personality of Godhead. Śrī Caitanya Mahāprabhu therefore says, guru-kṛṣṇa-prasāde pāya bhakti-latā-bīja: (CC Madhya 19.151) one must first please the spiritual master, and then one automatically pleases Kṛṣṇa and gets the strength with which to cross the ocean of nescience. If one seriously desires to return home, back to Godhead, one must therefore become strong enough by pleasing the spiritual master, for thus one gets the weapon with which to conquer the enemy, and one also gets the grace of Kṛṣṇa."