Not a teacher.
The phrase "grocer's port" is a new one to me. If you Google it you find this thread, the sentence you are trying to understand and one obituary using the phrase.
I imagine "grocer's port" was a cheap port wine. Nothing fancy. What might be called a "table wine."
("Any port in a storm," to make a play on words.)
The idea in the quote you reference is that having bad liquor is better than drinking water.
The analogy is that he would rather read bad literature than have nothing to read.
Student or Learner