Nichiren Daishonin wrote this letter at Kamakura in the first year of Kōchō (1261), about two weeks before he was exiled to Itō in Izu. Virtually nothing is known about the recipient, Shiiji Shirō, other than that he lived in the province of Suruga and was acquainted with two of the Daishonin’s leading disciples, Shijō Kingo and Toki Jōnin.

The title of this letter is drawn from a passage in the “Medicine King” chapter of the Lotus Sutra that speaks of “a ship in which to cross the water.” In this letter, the Daishonin teaches that the daimoku of the Lotus Sutra is the“ship” that can unfailingly transport one across the sea of life’s inevitable sufferings to the distant shore of enlightenment.

https://www.nichirenlibrary.org/en/wnd-1/Content/3