A great source for more obscure nautical terms is A Sea of Words by Dean King. It's a companion book to the Patrick O'Brian novels (which I recommended in the Book Recommendations thread) and really has some amazing stuff. Unfortunately, it somehow missed a few key phrases used in the books, most notably "a cheese of wads."

In spite of reading lots of fiction set during the age of sail, I haven't heard the term head lights. It may be more modern - but I'd guess it means the light in the head, aka the privy.