My answer is by no means meant to be canon, but more of a conjecture based on things we know about middle earth. Firstly, we should understand that Middle Earth is a high fantasy realm, it features several humanoid races, but also creatures unlike anything in reality that would be considered less real than myth. Creatures such as the majestic Smaug, Balrogs, giant Eagles and Wargs, as well as ghosts, undead and other ephemera. We know "the Watcher" is somewhat squid-like, perhaps modeled after a Kraken of earth mythology.
As we have in our real world, there must be a plethora of real water animals as we have which include everything from bacteria and virus, crustaceans and higher up the food and intelligence chain to copepods and cetaceans which we like to think are possibly as intelligent as us, and demonstrably at least as intelligent as a smart dog, as well as otters, seals and sea lions, which are also often likened to at least dog-like intelligence or better.
Since we know there is a layer of mythological ecology placed over our real world equivalence, there must be the same in the oceans. This part is at least evidenced by Gandalf's assertion that there are things in the deep waters.
There will likely be the water based equivalents of the ghosts and undead that haunt the lands, such as the ghosts of the Mountain Kings, and the Dead Marshes that almost took Frodo while Gollum was guiding them through.
Great battles of time past must have occurred over the oceans and there must be equivalent Revenants haunting those deeps. In the abyssal canyons, breed creatures as dread as Balrogs, or as massive as Oliphaunts.
And there might be creatures as mundane as mer-folk or some facsimile of them, who if the Ring were to come to one of them might become as corrupt as any land-based creature, and the Ring may return some day to its master.