In this passage we have two transmission errors that have been corrected in both the LDS and RLDS texts. The first error deals with the prepositional phrase “with sorrow”, which was lost as Oliver Cowdery copied 𝓞 into 𝓟. The 1840 edition restored the phrase by reference to the original manuscript. In his editing for the 1840 edition, Joseph Smith restored several phrases that had been accidentally lost when Oliver copied from 𝓞 into 𝓟. The other examples are (1) “the name of ” in the 1 Nephi preface, (2) “and partake of the fruit” in 1 Nephi 8:18, and (3) “for all men” in 1 Nephi 10:18.
The second error in this verse occurred when the 1837 typesetter replaced the indefinite article a with the definite article the in the phrase “into a watery grave”, thus changing it to “into the watery grave”. This replacement was probably accidental since either a or the will work in this phrase. In addition, Joseph Smith did not mark the change in 𝓟 when he did his editing for the 1837 edition. Moreover, the following (1840) edition restored the original a. The most probable explanation of what happened here in the 1840 editing is that when Joseph used 𝓞 to restore the nearby prepositional phrase “with sorrow”, he probably also noticed that 𝓞 read “into a watery grave” rather than “into the watery grave”. The RLDS text (which derives from the 1840 edition) has consistently maintained the original a. The LDS text restored the correct a in the 1920 edition by reference to either the 1830 or 1840 edition.
Summary: Maintain the readings of the original text in 1 Nephi 18:18 (the prepositional phrase “with sorrow” and the indefinite article a for “into a watery grave”); both readings are found in the original manuscript.