Waterwyrm

In the depths of the ocean, swelling to unthinkable sizes, dwells a species of foul creatures that drift among the currents and consume all manner of life that wander too near. Long rumored by old salts on shore but never spoken of at sea for fear of summoning the things, they are the worst nightmare of sailors who dare venture beyond the safety of the shallows.
Waterwyrms are gelatinous, serpent-like beings which can grow far longer than any vessel, seething in blasphemous coils as they crush and devour ships and sailors alike. They can extend and retract parts of their bodies into writhing tentacles which can quickly envelop anything they touch. While most feeding is done through what could be considered its mouth, a waterwyrm may pull its prey into itself with its tentacles, reforming its body around its misfortuned victim. A distant relative of sea cucumbers, waterwyrms can regenerate their genetic material, making them effectively immortal. This trait allows for continual growth, resulting in enormous specimens. Larger waterwyrms have no natural predators, allowing them to simply float about absorbing other living things while becoming even more massive. Fortunately, the largest of the species are too heavy to be supported by the sea floor, so they are unable to live nearer to shore. They are most frequently encountered far out to sea, around the region of soft-bottom soundings.
The ship known as the Teapot encountered a waterwyrm, as told in this account written by a crewmember.