When you say water is it the default water that is included in the map maker or actual water meshes?
If you're talking about rising and lowering the default water included in the map maker you simply increase or decrease the number in the area I circled here in red:

However if you're talking about an actual water mesh then you'd select the mesh and lower or lift the Y coords of the object by clicking the option I circled in red here:

This would allow you to lower the water down closer to the ground level.