Sewer Access is labelled in purple on the Fast Travel map, and they're hidden in alleys, sometimes beside empty lots. A lot of keep dungeons and temple catacombs have openings into the sewer. Any town with a river along it will have sewer tunnels empty into it, but entering that way is a drowning risk.
Sometimes you get quests down there for vampires or serpent men, amphibian men, olm men, fish men, etc. I've found some pretty good bits of gear in the dry tunnels. But exploring is long and tedious, and none of your quest targets carry anything close to a reward for the tedious bullshit you have to go through.