Actually, don't try uk.rec.scuba since Tobermory is in Canada and not
the UK.  Tobermory is the jump-off point for the Fathom Five Underwater
Park administered by the Canadian Federal Government.
Water is really cold right now (although people do dive if there isn't
too much ice).  Earliest I have been in is May 1 and the water, at 10
ft was about 34 degrees.  By summer some of the shallow wrecks are
quite balmy.  The Arabia is the deepest dive and because of the cold,
the most dangerous.  It is always just above freezing, no matter when
you dive her.  When you dive the deep wrecks you should add 10 ft to
your profile just to compensate for the adverse temperatures and lack
of light.  5 or 6 people have died on the Arabia.
Your wreck choice should be based on water conditions when you arrive.
Tobermory is where Lake Huron and Georgian Bay collide. There can be
bad fogs, rough water and big storms (hence all the wrecks). Other
times you can sit on a dive boat and look at the deck of a wreck 60 ft
below. There are shallow wrecks close to shore than can be dove in
almost any weather.  I would avoid diving the King, the Wetmore or the
sweepstakes unless you are socked in.  I would, if a boat is willing to
travel to almost Manitoloun Island, dive the City of Cleveland.
There is competition for space on dive boats on the weekend, but not
like it was ten years ago.  Usually there is space available.  Mid week
you can have the boat to yourself. No matter what, book ahead, it is a
long long drive and you don't want to take a chance on not getting the
charter you want.
The boats are good, there are two shops in Tobermory that carry
EVERYTHING and are  very good.   Check http://www.tobermory.org the
Tobermory website.  The Ontario Underwater Council has a good site too.
Hotels range from rustic to okay.  Some of the Inns are built for clubs
-- chummy but not much privacy.
Princess Hotel is good as is the Big Tub Lodge. J&B on the Rocks
(Cliffhanger & Tequila Sunrise ) is about a half hour out of town and
is built into the side of the  Bruce escarpment.
Serious wreck divers stay at Art Amos's Trail's End Lodge.  Its cheap
and cheerful.  crews from the National Geographic, Diver Magazinel,
HMCS Cormorrant, the Cousteau, etc, have stayed there.  Art is
Ontario's top wreck historian, when a ship can't be identified, Art is
brought in. Not sure when he is open, but, you can visit his website
at: http://www.trails-end-lodge.biz/