In his copying from 𝓞 to 𝓟, Oliver Cowdery accidentally omitted the pronoun I in the conjoined clause (“neither did I build the ship after the manner of men”). The resulting fragment apparently seemed odd to the 1830 compositor, who supplied the obviously necessary pronoun on his own. (There is no evidence that the original manuscript was consulted in proofing any of the early gatherings of the 1830 edition. 𝓞 was used in proofing gathering 22 of the 1830 edition, which covers pages 337–352 of that edition. For the evidence of this proofing, see Alma 41:8–46:30. Also see plate 11 of volume 1 of the critical text.)
Elsewhere in the text we always get a subject after “neither did”:
Thus the 1830 compositor was correct to supply the missing I in 1 Nephi 18:2. In this instance, 𝓞 is extant and agrees with his emendation.
Summary: Maintain in 1 Nephi 18:2 the subject pronoun I in the conjoined clause “neither did I build the ship after the manner of men”.