According to Sidney Sperry, in 2 Nephi 12:16 (cf. Isaiah 2:16) the Prophet prefaces a whole phrase not contained in either the King James Version or the Hebrew text. The phrase in question is and upon all the ships of the sea. With this phrase the Septuagint agrees, and it is perfectly easy to explain, on the basis of the Book of Mormon reading's being the original, why our present Hebrew text only has two phrases. According to the Book of Mormon the original had three phrases all beginning with the words and upon all. But a perfectly natural error---some scribe's eye inadvertently hit upon the second and upon all, and the first phrase was omitted. It is interesting to note that the Septuagint version has preserved the first phrase of this verse correctly, has omitted one phrase, and has corrupted another. The Hebrew has preserved the last two phrases correctly, but the Book of Mormon has preserved all three. [Sidney B. Sperry, "The Book of Mormon As Translation English," in Journal of Book of Mormon Studies, F.A.R.M.S., Spring 1995, pp. 212-213]
According to Victor Ludlow, it appears that the Book of Mormon contains the most complete retention of the original structure of this verse. Since the prophet Joseph Smith did not know Greek, and since there is no evidence that he had access to a copy of the Septuagint when he completed his Book of Mormon translation in 1829, this addition supports the fact that Joseph Smith translated the Isaiah portion in the Book of Mormon from a more authentic ancient text. [Victor L. Ludlow, Isaiah: Prophet, Seer, and Poet, p. 91]
2 Nephi 12:16 And upon all the ships of the sea ([Illustration]): 2 Nephi 12:16: A Comparison of Three Ancient Texts. [Victor L. Ludlow, Isaiah: Prophet, Seer, and Poet, p. 91]
Book of Mormon
(1) And upon all the ships of the sea
(2) And upon all the ships of Tarsus
(3) And upon all pleasant pictures
Septuagint
(1) And upon all the ships of the sea
(3) And upon all pleasant ships
Hebrew (KJV)
(2) And upon all the ships of Tarshish
(3) And upon all pleasant ships