In this version of my May challenge (I'm exploring different POV) the bench in the water would have caustics underneath on the leg where the sun reflects off the water, even thought the leg itself is in the shade, how would I get the reflections without the inevitable glow in the water from the light, is there a way to make an object invisible and still block light?

you could try to make the light really flat and then place it just above the water surface, to make it look like the light is actually bouncing of the water itself, instead of originating from a glowy light under the water. If that doesn't work you can always make one render with good looking caustics on the bench and one with goodlooking water and then use the maskrender and a 2D program to combine the two.

thanks, Rayraz, I seem to have posted this twice, my bad..., clay answered too. One of his animations has exactly what I was looking for...:)