sunlight

The sun's light inside the water — the two things a surface pass cannot draw, because neither of them lives on a surface.

The caustic net. Every pixel's world position is unprojected from depth, the sun ray through it is traced up to the surface plane, and the net is evaluated where that ray crosses. The pattern therefore lies on whatever is standing there at the right scale, and slides correctly across a curved white hull instead of being painted in screen space.

The shafts. The eye ray is marched from the lens out to whatever it hit, and at each step the same net is sampled where that step's sun ray crosses the surface. Light that got through the surface there is light standing in the water here, so the accumulation is the godray, jittered per pixel so its banding becomes grain.

Both fade on the same extinction the medium uses. strength is a uniform, and the pass returns on it before touching a texture or a derivative — so for the seventy seconds of DEADLIGHT that have no sun in them it costs one branch.