Ā-CUAUH-TLI, literally “water hawk/eagle,” a species of hawk, perhaps the Crane Hawk (Geranospza caerulescens) [FC: 41 Aquauhtli]: “It is of average size, not very large. The water is its habitat. It preys upon, it eats the waterfowl.” The best known fish-eating Mexican raptor, the Osprey (Pandion haliaetus), is definitively known as “Aitzcuauhtli,” literally “water Golden Eagle.” The Bald Eagle (Haliaetus leucocephalus), might fit the bill, but is both very large and rare south of northern Mexico. Two other medium-sized raptors are particularly partial to marsh habitats, the Snail Kite (Rostrhamus sociabilis) and the Crane Hawk, though both feed more on invertebrates than fish. Of these, I believe the Crane Hawk is more likely.
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 11 – Earthly Things, no. 14, Part XII, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1963); and, with quotation selections, synthesis, and analysis here also appearing in E. S. Hunn, "The Aztec Fascination with Birds: Deciphering Sixteenth-Century Sources," unpublished manuscript, 2022, cited here with permission.