Diving at Cancun has always been bad.  Cozumel diving is still great.

Cozumel has -a lot- of damage topside, but it is getting better every
day.  You will want to secure a hotel room or condo before you go.  I
would suggest someplace close to downtown.  Hotel rooms on the island
are limited at this point, but should be better by mid January, with
some resorts reopening -much- later than that.  There is much
construction and clean up going on - but if you are diving during the
day, you won't be bothered much by it.  Carlos and Charlies, Senor Frog,
the cruise ship piers are all wiped out right now.  The big grocery
store is still there.  Some restaurants sustained damage, but others are
operating.

If you are traveling with non-divers that want the big resort amenities
 and more topside activities, I would consider staying at Playa Del
Carmen and using the ferry to come over to Cozumel to dive.  Conversely,
they could go over to Playa Del Carmen and the mainland to shop or see
the Mayan ruins and such.  I talked to someone who used to work at the
Cozumel marine park (it will be rebuilt), who is presently working at
Playa Del Carmen, and he states they are up and running in a normal
fashion there.

I was on Cozumel last week via cruise ship, so only had a chance to do a
2 tank dive.  Right now most dive trips are leaving out of the Caleta
because the hotel dive docks got wiped out by the hurricane.  The DMs
admitted there is damage at some of the reefs, not unexpected.  But,
there is still plenty of the "good stuff" to see if you want to spend a
week or two diving.  I dove Palancar Caves at 77fsw and the visibility
in the swim-thrus was about as good as I recall from previous trips.
On our 2nd dive site at 60fsw max marine life was abundant.  We saw a
squadron of Eagle Rays plus others in pairs or singles, turtles, a
Spendid Toadfish, and all the other usual stuff.  Visibility here was
just a little less than I recall from previous dives, but it was still
as good or better than the viz we encountered in the Turneffe Atolls or
Roatan that same week.