Appears that this issue still exists after all and is not resolved from MC-129968 or MC-129625. I opened a world in same coordinates same seed as MC-129968. I previously played this in w20c, w19b, 16a and many versions prior. I'm confident that any others I open will be the same. macOS performance on the MacMini is finally better though (which should allow for basic shaders)(thank you team).
MC-131757
Random water behavior with blocks below sea level
Resolved
Awaiting Response
0
Looking at the versions you have played this world on, you've probably experienced the effects of MC-129265 before it was fixed in 1.13-pre1.