I didn't use a primivol solution but a number of repeated planes with the same transparency stacked above one another. Only problem with that solution is that you have a limit of say, 20 before Carrara's renderer calls it quits. (max ray depth) It worked pretty well also but this solution probably is better in that it isn't limited by the max ray depth. I'll have to keep this one in mind. You might want to add some Index of Refraction to the water surface.