In warmer climates they could but there probably wouldn't be enough food for them to thrive.
Crocodiles and alligators thrive in tropical and sub-tropical climes, they need warmth. They are large animals and need enough food to both expend the energy to get more food and to just survive. The sewers of most major cities aren't quite warm enough and only offer small rodents for the most part.
The urban myth stems from people flushing unwanted pets down the toilet. Because most septic systems aren't equipped to deal with large masses such animals would die and, if large enough, clog the system even if they are only very small babies.
Obviously any decent sized alligator or croc would never fit in a toilet.
Also obviously, such animals would have much better chances in any near by lake, pond, river, or stream and would leave the sewer in search of better hunting grounds rather than starve to death. If it's a cold place the sewers would be little better than said natural body of water. They are reptiles and, thus, cold blooded. They need warm environments to keep from dying of hypothermia.



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