average human is 150-200lbs. Think about it that way and it's a ton of meat. I don't know how much weight is bones, but at least 100 lbs of meat per person should be recoverable (on average.) A 30 lb turkey serves 12 people at Thanksgiving and gives leftovers for days.
A person is a LOT of meat.
http://en.wikipedia.org/wiki/1972_Andes_flight_disaster
16 people survived for 2 months on the meat from 29 others.
After reconsidering the episode I do think Bob was bitten in the water room. That might be my first legit nit about the show (why fight them down there instead of killing from above.) But regardless, there is no need to show the scene of Bob grappling with the zombie underwater unless he got bitten.
I don't quite know how it will fit in to the cannibals eating him. Seems odd that they'd die off in such a troll way. Also it's close to guaranteed Rick kills Gareth with that red handle machete like he said he would at the start of E01.