I did some experiments, but didn't find a suitable (less grainy) solution for PT water murk, while still maintaining a reasonably speedy render.
The sphere is obviously the most realistic option, but a (non-displacable) card certainly is faster. In these samples the water itself was colored by density 0.5 and color 0.3 (grey).
The fastest were water renders without integral murk, but with a preceding 50% (and 0.3 grey color) surface shader, again, faster on a card. But less sophisticated, and not even very much smoother.
Only with an AA of 12 did the grain get a bit less.