The sharks who dwell in the ocean do not care for the rivers which are gliding down into it. The devotees eternally live in the ocean of devotional service, and they do not care for the rivers. In other words, those who are pure devotees always remain in the ocean of transcendental loving service to the Lord and have no business with the other processes, which are compared to the rivers that only gradually come to the ocean.
Śrīla Rūpa Gosvāmī prays to his spiritual master, Śrīla Sanātana Gosvāmī, for the protection of Bhakti-rasāmṛta-sindhu—"The Ocean of the Pure Nectar of Devotional Service"—from the argumentative logicians who unnecessarily meddle in the science of service to the Lord. He compares their arguments and logic to volcanic eruptions in the midst of the ocean. In the midst of the ocean, volcanic eruptions can do very little harm, and similarly, those who are against devotional service to the Lord and who put forward many philosophical theses about the ultimate transcendental realization cannot disturb this great ocean of devotional service.