There's a lot of fun to be had considering sailors' and naturalists' thoughts on "sea monsters" through the 19th century, and human minds evolved to recognize patterns out of scrambled data (see also: Jesus toast). But even setting aside the problems with premodern SCUBA in the Arctic Ocean, beluga whales' sometimes protruding torsos do not make them, in general, the best candidate for They Might Be Mermaids.

Looking at ancient, medieval, and early modern descriptions of mermaids and their various mythological counterparts, the dominant emphasis on what makes a mermaid is the human top half and the animal lower half. It's usually a fish. Sometimes a serpent, as with the famous Melusine. Occasionally a slightly different idea of a fish because...because medieval art.

This is true in text as well as art. Here's a neat little article from 1877 collecting older tales of "mermaid sightings" that posits a connection to seals. If you scroll down a few pages, you'll see that the descriptions emphasize the human top half, from the waist up, and a piscine bottom half. From Henry Hudson's journal (always find the original source!):

This morning, one of our companie looking over boord saw a mermaid, and calling up some of the com- panie to see her, one more came up, and by that time shee was come close to the ship's side, looking earnestly on the men : a little after, a sea came and overturned her : from the navill upward, her backe and breasts were like a woman's, as they say that saw her ; her body as big as one of us ; her skin very white ; and long haire hanging downe behinde, of colour blacke : in her going downe they saw her tayle, which was like the tayle of a porposse, and speckled like a macrell. Their names that saw her, were Thomas Hilles and Robert Rayner.

(Notice additionally in the Gentleman's Observer article, which has modernized spelling if you're not used to archaic, that Proctor would be willing to accept Henry Hudson's observations at face value, but certainly not the observations of his crew.)

So at least on the European end, it seems as though sailors took the top half of the mermaid as identificatory, not human-like features in the bottom half. Beautiful famous statues notwithstanding.

But of course, European mermaids aren't the only mermaids in the world. And while we can trace the genealogy of mermaid myths/folklore back to the ancient Near East, that's still not who would be the most likely to encounter them. So I want to take some time to visit Thule (Ancestral Inuit and Yupik) mythology--and how it changed with European contact.

One of the major figures is usually known as Sedna, though modern nations in different areas might have other names. Sedna is the mother goddess/sea goddess/nature spirit, and in the Thule creation myth seals and whales are born out of her body. However, in recorded versions of the oral tales, Sedna is a human woman living an archetypal (if sad) life who ends up below the sea. Descriptions of her focus on her severed fingers, her hair, her beauty.

However, in modern northern art? Sedna has become a mermaid. Studying some early contemporary depictions of Sedna, from the 1950s on, Carol Prokop argued that depictions of Sedna as a mermaid represent a new take on the story, not an illustration of centuries of Thule belief. She posits that mermaid-Sedna is a "composite form" not just bringing together the human and natural worlds, but also the original 1950s artists' attempt to reconcile the cultures clashing into each other around them. They recognized the European mermaid as fulfilling overlapping functions with the Sedna they still knew stories of, a point of contact

The point overall remains: the people in a position to have significant contact with beluga whales were not on the lookout for mermaids. The people on the lookout for mermaids were looking for human faces and torsos, not legs.