This entire lake was filled with blood. The souls of the dead that are there are put to use with their bodies added to the sea of the blood of the damned. The sea is also bottomless; those who fall in it can never escape.
This is the second stage in Mortal Kombat 11 to have a normal and alternative Stage Brutality associated with it; tossing the starfish at the opponent causes their head to fly off and the starfish to fix itself in its place.
Those that traveled the sea through boat were often challenged by bloody Undead skeletons that boarded their craft and attempted to kill the living.
Erron Black's Arcade ending has him dropping Kronika's hourglass into the Sea of Blood, so that no one would ever gain it and use it.
The Terminator's Arcade ending has him willingly enter the Sea of Blood in order to prevent anyone from finding Kronika's hourglass.