I just noticed something.
Is the first image you uploaded an alpha? Because I can see the rock within the water and I shouldn’t be able to.
I don’t have the arnold render so this may not be fair test. Cooking that bad frame right now to see what I get.
I thought it might have been the displacement on the water getting too high and the camera was in a wave for one frame but the displacement doesn’t look deep enough.
How long does a frame take with arnold vs the physical.? (more just to see if its worth getting – doing this with a two thread physical is painfull)
And now that I’ve had a look I see why I can see the rock – there is transparency on the water. You actually don’t need that if you aren’t going to see through the water and the waves are so low it’s not going to show. But it would speed up the render.