When the Lamanites finally took the city of Desolation, the Nephite armies found refuge at the city of Teancum "in the borders by the seashore" (Mormon 4:3). According to John Sorenson's Mesoamerican theory, the location of Teancum could be around the site of Pilapan, some dozen miles away from the modern-day Minatitlan, Mexico on the Gulf coast. [John L. Sorenson, An Ancient American Setting for the Book of Mormon, pp. 345-346]
Joseph Allen places the city of Teancum relative to modern-day Acayucan, Mexico, his proposed city of Desolation. Acayucan is about 28 miles from the Gulf of Mexico. [Joseph L. Allen, Exploring the Lands of the Book of Mormon, p. 232].
According to Richard Hauck, "the west sea (or Pacific Ocean) must have been adjacent to the city of Desolation, for during a major battle in A.D. 362 the Lamanite dead were "cast into the sea" (Mormon 3:8). By comparing his cover maps, I presume the city of Teancum was somewhat to the northwest of the [modern-day] city of Arriaga, Mexico near the [Pacific] coast. [Richard Hauck, Deciphering the Geography of the Book of Mormon, p. 188, inside front cover, inside back cover]
[See Geographical Theory Maps]
Geographical [Theory Map]: Mormon 4:2 The Lamanites Take Desolation (364 A.S.)