The phrase "their dead were cast into the sea" (Mormon 3:8) might imply that the battle site, and also the city of Desolation were either close to a sea or that they were located at a point on a river or other drainage into the sea such that it was close enough to be considered "the sea."
According to John Sorenson's theory, if the location of the City of Desolation was on the Coatzacoalcos River (Setting, p. 345), then the dead "were cast into the sea" presumably by way of the Coatzacoalcos River which empties into the Gulf of Mexico. On the other hand, according to Palmer (Cumorah, p. 33) and Hauck (Deciphering, pp. 102-104), the location could have been by the west sea (Pacific) near Laguna Zope.