From page 328 of the 2011 translation of Cotton Mather’s Biblia Americana vol 1 we find the following question and answer:
“Q. What are the Creatures intended in the Original, where our Translation reads, Great Whales? V.21.
A. Job Ludolphus ha’s demonstrated, That the Name, Tanim, signifies, Crocodiles. (Compare Ezel. 29.3. and 32.2. where, tis certain, there can be meant nothing else.) And we know, that Crocodiles are elsewhere singled out, as a most astonishing Work of God. cont…. “
The verse of interest here come from the King James Bible, Genesis 1:21: And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good.
Here we have one of many cases from the King James Version where the translators used terms of animals and plants that were well known to the interpreters at the time and they did not consider the flora and fauna with which the original audience would have been familiar. The same thing is happening today, creation scientists frequently interpret/translate the word Tannin as dinosaur or in the case of Genesis 1:21 the great sea monster as a long-necked plesiosaur. A plesiosaur or a whale both are translation that are affected by a modern worldview. Mather would not have been familiar with dinosaurs but neither does it seem likely that the original Hebrew audience would have been either. Nor does he simply go with the King James use of the world Whales but is convinced of an argument which uses an animal for which the Israelites would have been quite familiar. With respect to the original audience, crocodile or hippo would seem the most natural interpretation as to the what great sea monster was in mind with the text left translated just as “sea creature.”