i hear what you are trying to do - and of course, you can do what you wish.
I have tried water shots and I am not sure that 'tack sharp stop motion' is the my favorite approach. You/I never see it that way in real life. I don't know that a creamy fall is what I would go for either. Using the link above as a guide, I like the 1/15 second effect.
a suggestion is to set the exposure for the waterfall - at 1/15th ss use the SBs to catch the people still (at TTL - they should still be properly exposed) without motion due to the short duration of the flash.
frankly, I would try it at 1/15th and also at 1/30th.
the desired result - natural looking falls and well exposed people.