As the will of the Buddha, no sūtra is superior to the Nirvana Sūtra. Seven kinds of people are enumerated in this sūtra. The first are people called icchantika, who are unable to get out of the ocean of life and death just as a huge rock thrown into the ocean. It is too heavy to float in the water and remains at the bottom of the sea forever; they are eternally drowned.

The second kind of people are those who are able to float in the water only momentarily. They have the power of floating in the water, but they cannot stay above the water because they have not learned how to keep floating. They are not icchantika but similar; they, too, are eternally drowned.

The third group of people are able to stay in the river of life and death without ever drowning. They are at the rank of śrāvaka such as Śāriputra. The fourth are the people who learned how to float in the water but did not find the way out of the water. The fifth are those who can stay above the water to look around, but got scared, and quit.

The sixth kind of people are those who float in the water, are afraid of pirates in the distance as well as nearby, and decide to stay in a shallow place. The seventh group of people are those who have reached the other shore, where they enjoy great pleasure without fear. Those of the fourth, fifth, sixth and seventh groups refer to pratyekabuddha and bodhisattvas.

Jōren-bō Gosho, A Letter to Jōren-bō, Writings of Nichiren Shōnin, Volume 5, Page 168

Imagine learning to float but getting scared and quitting, or worse staying in shallow waters for fear of pirates. Only with the Lotus Sūtra can the rock float to the other side.