What is something coming from Bilgewater called?

I typically go with Bilgerat.

Bilgerat?

I imagine they're called "scum" most of the time.

If I had to guess, a more formal name would be a "Bilgewatite." It flows a little nicer than most other variations.

Pirate.

Or maybe they ignore the water part so Bilger... Bilgian.... Bilgite.

Bilgewatian is correct imo