Here's the things I didn't understand from this otherwise perfectly logical movie:
The subway tunnel turned into a river, and yet the subway cars continued to run. The electrically-powered subway. Wouldn't this have blown a fuse somewhere?
How was Tara Reid operating her buzzsaw? Again - ELECTRICAL. Yet there was no plug in site.
Finally, there is no way that was supposed to be her arm at the end! As Scott and many others have pointed out, Tara's hand was severed at the wrist. That arm at the end went up to the elbow. Unless Tara had a crazy long arm that we never noticed, it's not hers.