new, so time for stupid questions

I asume it is not posable to have terranes with minium hieght less then 0; for water? Or what hieght does worldmachine figure water at?
I’ve been trying to use hieght spliter to make a masks for sub surface rocks for lakes/rivers, but I am not sure what hieght to set my water at in terragen.

Correct; the lowest possible height is 0. WM doesn’t really know anything about water at all; you can create the base of your terrain at any height you want and put the water level there in terragen.