The ending *spoilers*
Watching this again recently, I have this theory that the young girl at the end wasn't really there, she may have been a figment of Marcello's subconscious trying to remind him of his innocence in a last-ditch attempt to get him away from the upper class
http://www.youtube.com/watch?v=SyqsExlstZE
From 2:40 there is an establishing shot showing nobody further down the beach which lasts for 11 seconds. 13 seconds later at 3:04 the young girl is seen with a clear stretch of beach separating her + Marcello. The next shot at 3:11 shows the same girl, only now a pool of water has magically appeared inbetween Marcello and her. But it's not just the young girl who has suddenly appeared, a couple of kids and another person dressed in black are playing in the water even further in the background (they weren't seen in the earlier establishing shot either).
So did Fellini make a large continuity error + I'm reading too much into it or does my theory hold some weight?
Marie got her ass kicked in "Au Hasard Balthazar"