Well, I think I found my answer, and it's the very anti-climactic "depends on the water world."
A "shallow"(say, 10km) water world might be just as susceptible to tectonic effects as any terrestrial world, but that would change in deeper ones.
According to this, beyond 632.4 MPa of pressure(which you would get at depths of about 63km,) water would be crushed into a form of ice, ice-VI, that can stay solid above generic water's melting point, and past 2.1 GPa(210km,) it would become ice-VII, which you couldn't even
cook back into a liquid form.
So, a sufficiently deep water world would have a thick, regenerating layer of unmelting super-ice around its lithosphere(if it has one,) which I'm pretty sure would prevent any kind of tectonic activity.
Scientific naming aside, this effect is actually acknowledged in canon in Volturn's description:
Volturn possesses a thick atmosphere over a world-sea which has been seeded with re-engineered Terra-type life forms that have largely displaced the relatively primitive native organisms which once formed vast drifting colonial mats. Beneath the hundreds of kilometers of ocean a core of water-ice over a rocky-metallic core is formed by the immense pressure. Volturn's population lives in a diverse range of floating habitats and subsists largely on farming and processing complex organic compounds.
As well as the generic water world description:
A primitive atmosphere covers a world-ocean hundreds of kilometers deep. Its waters teem with primitive life and vast algae-analog mats can be seen by instruments from space. A core of water-ice over a rocky mantle is created by the terrific pressure of the sea.
I'm a bit conflicted. If I assume that a water world is shallow, it
does make sense for it to be affected by tectonic activity, but that would mean ignoring the generic description, which implies that they're
all deep enough to form higher forms of ice and thus subdue the unruly rocks below. But if the hazard can really spawn on water worlds naturally, then I don't even know what to make of this.