I will be delighted if someone can come forward with primary source material from the period that can shed light on this. Drawing from nineteenth-century folklore collections, it is possible to draw some conclusions and to imagine a few more.

Sailors have their own distinct occupational folklore. At the same time, the general population had/has its own folklore about the sea and the ocean depths. Some of these traditions overlap. One of the peculiar things about maritime traditions is that they are often held by the general population and these are often about the liminal zone defined by the tides and extending a ways - but not too far - into the sea. This is where many maritime legends are set: most mermaid stories involve encounters in the tidal zones. My understanding of things like water cows and water horses is that they are usually encountered by regular people (rather than sailors) and they encounters usually occur near the shore (or even in lakes/rivers).

Sailors had ever reason to fear the very real dangers of the broad expanses - and the depths - of the ocean. They populated the depths with all sorts of large, terrifying monsters, but my understanding is that the actual forms of these creatures were often vaguely considered. At the same time, sailors shared the terrestrial folklore, and their stories of mermaids and kelpies were usually restricted to near-shore encounters.

It is an odd thing that the idea of aqua-horses is widespread in European folklore. They drew Neptune's chariot; they lived in Scandinavian lakes where they lured children to be drowned, they galloped out of the control once a year to drown their rider,an annual tribute that the water demanded, and they appear in various Scottish stories as kelpies. Wherever one encounters water horses, they are typically dangerous, and they often seem to determined to drown people.

Given all of this - and drawing on nineteenth-century material that gives us only limited perspective - I think we can conclude that sailors of the Golden Age of Sail would recognize that there were a variety of water horses, and that they were dangerous, wanted to drown people, and were most often encountered near the shore.

Let's hope for a better historian with sources from the period. Until then, perhaps these observations will help.