In the game, there’s indeed nothing
In the lore, there’s supposedly a bottom at 1600 to 2400 meters deep, but there’s nothing said about what’s actually in there besides plankton
Subnautica is probably the game that does the best job at keeping players from exiting the map; Because if you do try to exit the map, the edge cuts off suddenly, the water gets suspiciously blue, and you don’t know why it’s happening until a ghost leviathan appears