To properly help you, we first need to know more about your body
style... Please post nude photos so that we can more properly evaluate
your true needs...

<dirty-old-man-grin>

Seriously though, it's going to depend upon your cold tolerance... I
suspect that you'll need a minimum of a 3mm full suit or perhaps even
up to a 5mm if you are particularly cold natured... The Farmer
John/Jane style will be warmer since you double the thickness on your
torso due to the overlap...

Shorties are available for rent at several places around the island
during the summer, including the snorkel-only Lover's Cove and the
Casino Point underwater park and the beach club.  Normally shorties
are what they rent to snorkelers, since you'll have the sun on your
back and not be totally immersed like a diver.  Water is still really
warm in September, though island amenities start to fade away after
Labor Day and it's quite possible that it could be harder to find a
rental suit right at the snorkel site.  All that means is that you'd
have to go to the Catalina Divers Supply on the green pier, or Scuba
Luv in town.

I'm guessing that in late September, water temperature will be right
up near 70 degrees on the surface, if not a degree or two over.  The
jetski suits are probably too light, as they're mainly for spray or
for brief immersions.  A fullsuit might be too warm with the sun
beating down on you, and possibly too cumbersome for comfortable
snorkeling.  I'm not sure of the thickness of the rental shorties, but
they were fine for my two aunts a few summers ago.  Both of them are
in their sixties and thin.

I'm a strict novice, never been diving, working on my certification, but
have been doing a lot of research into wetsuits and such.  They do make
wetsuits using two thicknesses, say a 3mm/2mm.  Something like this would
give you a bit more freedom to move, yet cover you completely.  Most of the
suits I've seen on the net are based on water temperature, so your suit
should match to that, and, as another poster said, your own reaction to cold
water.

If the water is really warm you can get lycra shirts (rash shirts) or full
lycra suits.  Mostly to protect against sunburn and maybe a sting or two.

My girlfriend never gets cold and sleeps with all the windows open in below
freezing weather.  She's certified and I don't think she's ever worn a
wetsuit.  I'm the exact opposite.

I assume that you mean 5 ft tall instead of 5 inches tall... Otherwise,
you just need a very wide and short koozie... Maybe a very wide version
of this:

http://www.dollarimprint.com/koozie/neoprene-wetsuit-rubber-bottle-jersey.htm