there's nothing to do in there, both just have some empty event triggers, but they're registered as areas in the game files and you can visit them using mtool. I'm pretty sure they're both just completely black though. if memory serves the sewers have an image or two?
there's an unused (so far) woman that appears both at the sewers in the hotel, below and to the left of the school by the river, the hidden shrine, the inaccessible maze, and there might be one or two more that I'm forgetting. she also has a cool knife for you!