I recently came across an Internet blurb by Dr. Harold
Katz, a dentist who heads the California Breath Clinics and
author of the Bad Breath Bible. Having been in the dive business
forever, I've never encountered a diver with the pariah's
affliction called "Diver's Mouth," but perhaps the dive boats'
exhaust fumes have prevented me from noticing.
Katz writes: "Some people who take up scuba diving find
  that, after a few weeks, they suffer from jaw aches and halitosis.
  What is going on here? Can anything be done about
  it, short of finding a new hobby? The good news is that no
  one needs to quit scuba diving just because it gives them bad
  breath. In fact, there is a name for this condition - - 'diver's
  mouth syndrome.'
"Several studies appearing in the journal Dental Update have addressed diver's mouth. Novice divers choose a mouthpiece
  that is too small for their teeth and jaws, researchers
  say. As new divers swim underwater, they clench their teeth
  into the rubber of the mouthpiece, both out of excitement
  and because they're towing an oxygen (sic) regulator with
  their mouths. Biting into a poorly-fitted mouthpiece can
  cause jaw aches, and lead to nicks in the gums. These small
  cuts may get infected, leading to oral odor and gum pain.
"Besides choosing a better-fitting mouthpiece, individuals
  with diver's mouth may consider using a specialty breathfreshening
  rinse that contains no alcohol or sodium lauryl
  sulfate. Such products kill bacteria without further irritating
  the gums."
So, whenever you open your mouth on a dive boat, and
  your fellow divers jump overboard, you, my friend, may have
  the dreaded Diver's Mouth. Beware.
- - Ben Davison