Here's a thought: We are all just assuming that the potion Dumbledore drank was the potion that was there when R.A.B. took the locket. Why? In the book it was never stated that the basin re-filled, so it may still be empty now. And we knoww that somebody was there before, and that they had to get to the bottom of the basin. Are we just supposed to think that this person happened to have the exact same potion in his pocket and replaced the locket with the fake one and then added in the potion? Who knows what kind of potion was in the basin originally? R.A.B. could have replaced the original potion with the green potion, there is no reason, as far as I can see, why the original potion could not have been completely different.
So one person may have been able to get past the original potion.
Also, If there was more than person going into the cave the first time, one of them must have either been an underage wizard, a squib, or a muggle. Dumbledore said the boat would take one full-fedged wizard across the water. So if there were many people, there would be only one wizard.