There are lakes in the ocean called brine lakes/pools. Brine is essentially concentrated saltwater; its high salinity means it's denser than water. On rare occasions, brine doesn't mix enough with the existing saltwater around it, sinking to the bottom of the ocean and forming these lakes. The lake itself is usually devoid of life; brine itself is so salty that animals go into toxic shock if exposed for too long. However, the edges usually are full of life, where usually things like mussels and other extremophile organisms thrive.

Side note, subnautica's lost river is based off of this. No big leviathans in real life though, at least none observed yet...

Video for fun: https://youtu.be/ZwuVpNYrKPY