So, water bodies work fine as long as I do not create a landscape from a height map: when I do that, I get the water bodies circled by an odd wall of water, and I can’t understand why that is happening. See the attached screenshot. Any ideas?
Hi,
Looks like your water take information from the Height map. Is your water part of the same Landscape than your ground ?
Nope, it’s on the Water layer.
Oh, well then… 5.1 fixed the issue, yay! 
Well… I cannot upgrade that project due to a specific requirement 
Oh, well that is a very bad news I guess Can’t help you further then, sorry!
Ensure that the landscape spline’s points do not extend past the physical landscape. Having the spline verts outside the landscape can cause issues like this.