Depending on the lighting available (and if composite images in photograph are not your thing), you may be able to get both in one shot. If they are adults, and you clue them in that they need to stand still, AND if you are willing to take several shots to get the best one, you might be able to get the shutter speed down low enough as to smooth the waterfall while still getting sharp people.
I tried to find some examples but had discarded all the really fast shutter speed, but this might give a feel.
The first is 1/40th second, the second is 2.5 seconds. The latter is more typical of what people expect a waterfall image to be. But the former is not completely "frozen", it has enough movement the whites get a bit of a smoothing. If you got down to say 1/15th or so, and it's a decent speed fall, you may well get enough smoothing.
Whatever you do, if you have a patient audience, take a lot of shots. I'd set the camera to bracket and just shoot bursts with each shot, at least a burst of 3 if not 5. Then shoot with a few different settings. And if you have to fall back to photoshop to combine, you should have plenty of material.