Here the original text read “to the borders by the seashore”. In the 1837 edition, the preposition by was accidentally replaced with of. The 1981 LDS edition correctly restored the earliest reading (as found in the printer’s manuscript and the 1830 edition). As argued under Ether 14:12, the original text allowed for only the phraseology “to the borders by the seashore” (that is, whenever there is motion “to the borders” defined with respect to the seashore, the preposition is by). The earliest reading here in Ether 14:26 supports that claim. For three other instances in the history of the text where “the borders by the seashore” has been replaced with “the borders of the seashore”, see under Alma 50:25.
Summary: Maintain in Ether 14:26 the preposition by in the phrase “to the borders by the seashore”, the reading of the earliest textual sources (𝓟 and the 1830 edition).