Continuity mistake: In the first 'Ice Age' Diego isn't afraid of water (in the beginning he chases the woman and the baby through the river before she jumps off the waterfall and never shows any fear) but in this one he suddenly is. It's not like that he is just afraid of swimming because in the second movie he shows fear anytime when he touches water.
Continuity mistake: At the beginning of the film you'll notice that when the blue creature from the ice chunk dives back into the water he has a tail fin. When the creatures attacks Manny and his gang and Manny shoves the creature off it dives back into the water again without a tail fin. Then at the end when Manny is attacked for the last time the blue creature has its tail fin again.