Deep Sea

The Deep Sea is aptly named. It is far deeper than the surrounding ocean but no one has been able to measure it. It is also fresh water but no one knows how since it is fed by the ocean. The water coming in contains salt and the area near the mouth is salt water but the salt seems to just disappear once you get further in. There are more theories than people that care but they all agree it is not natural.
Most of the civilized world is centered around the Deep Sea, and it facilitates trade between kingdoms. There are plenty of monsters but most are not big enough to threaten anything larger than a skiff.