Update: Here is the answer from a year ago: How to reduce the water level on Z-axis? - #3 by jRoogles
Clarification, I am working with a lake, and In the Asset: Water_Material_Lake (material), and also found this setting in the associated Material Instance, under the Cat: Global Static Switch Parameter Values, go to “Use FixedZ” and check it. This seems counter intuitive, but, now, the control handle for the Water Body Lake will move the water level up and down on the Z level. I’ve not tried it with an ocean, but fingers crossed this works for you. 
I would think, that this is a setting that most people would want on by default. 