I have tested this on Minecraft version 1.21.9 with Fabric installed, and found that installing both mods simultaneously causes water to fail to render, appearing as if it has vanished.

PhysicsMod uses default settings in all tests.

If players encounter water not rendering, please check for the ImmediatelyFast mod.

haubna/PhysicsMod#1115

From my testing this seems to happen whenever one of my custom particles is about to render (splash, rain, snow, dust). Not sure why though because the splash particles for example should be identical to the way a note block particle renders. My custom particles also just don't seem to be rendering at all (they use the TRANSLUCENT layer but switching them to OPAQUE didn't seem to make any difference).

EDIT:
Did some further testing and apparently it doesn't like it when I bind my custom FBO.

Switched my custom FBO implementation to use GlStateManager._glBindFramebuffer instead of GL32C.glBindFrameBuffer. This seems to fix it (probably due to some Mixin). Issue can be closed.