Parts are still affected by terrain’s water buoyancy when collision groups are set to prevent collisions. This bug is pretty straightforward and happens 100% of the time:
My use case involves implementing custom water physics and generating water graphics client-side for nearby water to improve mobile performance; this is significant for deep expansive oceans, and also shaves a few megabytes off my save file. Generating water graphics client-side also enables me to potentially perform water cut-outs client-side for nearby boat hulls. The short default water fog range is also a limiting factor when trying to create large underwater worlds.
There are other use cases where developers would benefit from precise control over water physics, and perhaps the ability to disable water while retaining collisions from solid materials.