According to the answers to Would a mermaid be kosher?, the Gemara and poskim apparently believed that mermaids exist, and would not be kosher.
So, if these mermaids and sirens exist, where would they be found? (Please do not tell me ♪ under the sea ♪.)
Dr. Leiman writes on the mouse that is 'half-mouse-half-earth" that, (p.452):
"... it comes as no surprise that the rabbis discussed the status of a creature they had never seen, and one that modern scholarship would label as imaginary. The greatest scientists and historians of their day took its existence for granted. If so, its halakhic status needed to be discussed and clarified."
In the same articles Dr. Leiman quotes Sherira Gaon, Maimonides, Abraham Maimonides, and R. Samson Raphael Hirsch, all of whom admit that, to some degree, the Sages worked with the science and mathematics of their times, and that things they state in those fields should be considered in that vein.
Mermaid legends in the Near East began as far back as 1000 b.c.e. The Sages heard of such a creature, and felt that they needed to discuss it during this veterinary gloss in Bechorot.
So to answer the question, just because the Sages discuss a creature doesn't mean it exists, and can be found. It is possible that mermaids don't exist, and can't be found anywhere, and that the Sages were working with bad intel.
The Gemara has a lot of philosophical discussion on what the halacha would be in a hypothetical situation that in reality probably never happened but is discussed for the sake of Torah and Emes.
So they were discussing the halacha of whether a mermaid is a kosher fish because it has fins and scales or whether it is a non-kosher mammal because it lacks the kosher signs of the cud and hoof.
They were expert in halacha, not science. Their halachic ruling is impeccable even if it would never actually apply in reality.
Rabbi Slifkin addresses Mermaides, concluding that they are probably dolphins, in a post entitled "The Wisdom of ArtScroll" (for once, not sarcastically) which he features in his book Sacred Monsters.
As a summary he writes:
"In brief: The Gemara provides a perfectly accurate account of dolphins. Rashi, however, for reasons that I discuss in my book, (mis)understood the Gemara to be referring to mermaids. Which, in the view of most (but not all) people, do not exist."
He then quotes the Artscroll:
“There are marine animals,” writes Rashi, “half of whose bodies are of human form, and half in the form of a fish. They are called sereine in French.” Rashi clearly refers to mermen (the French sereine derives from Latin siren, meaning mermaid), whose existence was widely accepted in the ancient and medieval world and indeed until recent centuries. (According to Raavad in his commentary to Toras Kohanim 3:7, sirens are mentioned as well in Toras Kohanim ibid.) As understood by Rashi, then, the Baraisa teaches that humans and mermen can interbreed.
Others suggest that the dolphins of the Baraisa are none other than the familiar dolphins of the order Cetaceans. These endothermic (warm-blooded) air-breathing mammals “reproduce as do humans” (following the variant kbnei adam) in that they copulate ventrum to ventrum (the manner ascribed to humans later in the Baraisa), bear live young, suckle their calves, and rear them intensively for six or seven years, to near adulthood. Dolphins were known by very similar names in the milieu of the Baraisa: Latin delphinus, from Greek delphis. Delphis is related to delphys, meaning womb, so that the genitive delphinos probably denoted [a sea creature] possessed of a womb; the very name dolfinin thus suggests that the animals in question “reproduce as do humans.” Rav Yehudah may have called dolphins sons (or people) of the sea because of their affinity for humans (they commonly approach and accompany boats), and because they often evince humanlike intelligence in their behaviors and social interactions.
So to answer your question: mermaids are actually dolphins. Primarily found in the sea.
The National Oceanic and Atmospheric Administration themselves have said:
Scientists estimate that 91 percent of ocean species have yet to be classified, and that more than eighty percent of our ocean is unmapped, unobserved, and unexplored.. While these statistics may sound daunting, they have not stopped the global scientific community from striving to amass as much knowledge as possible about ocean life.
Since scientists say the vast majority of the ocean has yet to be explored, the answer may simply be that mermaids are out there among the unseen 80% of the ocean.
