The sewers are mainly below the streets of Slagtown, where deformed chuds and nicotine addicted chobos go around beating people to smith and takin' their lighters. Be careful, if you venture too far down, you may come across a beast known as the Charnold, the Kraken, or the dangerous bug queen who could and possibly will consume you. The three of which, when you are newer, can massacre you. But, later on, the sewers will be a fun and mildly challenging place to train your skills and collect some chudskins. Not to mention if a shortcut to places if you play as a chud, abomination or vampire.
There are also Crocodiles, Alligators, and Caimans in the parts of the sewers that stretch under Freedom City proper, they instantly aggro you and are quite harder than chuds and chobos.
A detailed map with descriptions can be found here.