If there's no ceiling fan, there are plenty of places where lack of A/
C could be extremely uncomfortable if not deadly.  In a cramped
liveaboard cabin it's very necessary.  Unfortunately I've been on
several boats where the A/C is positioned to blast directly in one's
ears while sleeping, and that can definitely make at least one diver
congested.  I've squirted plenty of Afrin up my nose in direct
response to too much overnight A/C.

But other things can mess up your head too.  I read somewhere that
eating dairy products increases mucus formation, so I swore off dairy
on dive trips, ordering my post-dive cheeseburgers without cheese, and
drinking my coffee black.  Didn't seem to make enough of a difference,
thank god, so I threw that idea out.

Boat exhaust can be a killer, and sometimes hard to avoid if you're
seated by the engine.

The worst for me now is plane travel.  I've ended up with a cold a
couple days into a trip that can only be timed with my flight there.
Lately, I've been fine, but now Janna gets the cold first and then
sure enough, a couple days later so do I.  My next flight, we're gonna
wear masks, I swear.

The codeine cough syrup is out?

For the average person without any contraindications, a diving ENT may
suggest one or both OTC remedies: pseudoefedrine (i.e. Sudafed) and/or
oxymetazoline (i.e. Afrin spray).  Both work, great in combination,
but both have side effects as well.

Sudafed can make you wired (bad if you need your beauty sleep in order
to catch an early a.m. dive boat, but nice for a fiesta night) and it
can wreak havoc with blood pressure.  The latter effect is
particularly of note for scuba divers where the major cause of death
is heart attack or stroke from over-exertion.

Afrin has a "rebound effect" which means that when it wears off,
you're back where you started, or actually a slight bit worse than
where you started.  That means you have to squirt it again the next
day, and the next, and so on.  To some, this leads to addiction; to
others, it means you at least have to squirt it every single morning
of the rest of your dive trip.  Unlike easily popping a pill like
Sudafed, the nasal spray makes you cough and gag as the bitter fluid
runs down your throat, and it's best not done in mixed company at the
breakfast table, and preferably with lots of Kleenex at hand.

There are likely other side effects of both drugs, which is why the
cautious may wish to get the advice of a doctor, but these are OTC
solutions and therefore self-prescribable.  AFAIK, in the normal case
of diver congestion, the ENT won't recommend any more than the above.
At least that's what my ENT recommended.

(Murray Grossan, a diving ENT who happens to be based out of the
hospital where I was born, makes a product called Clear-eze.  He used
to spam them here, so I was forced to boycott it and hence haven't
tried it, but it's a natural enzyme-based product that he swears
works.  I don't know if he and his product are still around, but
that's yet another choice.)

A final warning about the decongestant drugs:  one side effect that
pretty much affects divers alone (but it can happen on an airplane as
well) is the reverse block.  That is, the decongestants can wear off
mid-dive, and make it near impossible for your tubes to equalize back
in the other direction as you ascend.  This can be a major PITA (or
PITE to be precise) because it's a bit too late to abort the dive at
that point.

Also, any ear trauma you suffer during a dive trip may haunt you on
the plane ride home.  If you need meds to dive, it might be a good
idea to use them before take off and landing.