5mm won't be enough for you, but a dry suit certainly isn't necessary
either.

The major demand might be getting back into the inflatable dinghys after
doffing your gear.  The Aggressor's dinghies had little ladders if
absolutely needed but it was considered quite unmanly to use them.  As a
diver, you wouldn't want to be considered unmanly.

Otherwise, aside from the colder water, viz is limited, though not really
bad, and currents can be strong and unpredictable.  Surface conditions can
also be a bit rough.  From the perspective of someone who's dove worse
conditions in much, much colder water, it was a cinch.  On the other hand,
it might be a WWW's idea of hell.

Having been to the Galapagos, and many other places besides, I would
offer the following advice:

1.  If you are prone to cold, get your hands on a semi-dry 7mm wetsuit.

A drysuit would be overkill.

2.  If on a liveaboard, i.e. doing Wolf and Darwin, water is warmer up
there,
so wear your wetsuit hood OUTSIDE your mask strap - put on mask
first.
This will allow you to stay cool in the warmer surface waters, and
give
you the advantage of pulling up the hood for extra insulation when
you
pass through a thermocline. Vice versa on the ascent.

3.  Demanding? Yes! The currents are strong, and sometimes flow away
from
your intended underwater destination. You will, at some stage, have
to
swim against them. Drift diving is not commonplace there as such,
if the
current is going your way, great!

4.  Weight yourself properly!!!!! I cannot emphasise this enough! If
you go
with a 7mm, then it will be impossible for your buoyancy to be
neutral
at all depths without some compensation for suit compression on
descent.
IMO it is imperative that you be neutral at 4m on the ascent, that
will
mean you will use the minimum compensation for buoyancy loss
throughout
the dive. Why? Read on ...

5.  As the last poster has said, currents are unpredictable there. That
means
they can go up and down as well as horizontal. If caught in a
downcurrent
you will need all the lift you can get from your BC. If you have
already
used half the BC's capacity to compensate for extra lead, then that
will
leave you with only 50% lift to help get you up against that
downwelling.
Hence the need for accurate weighting.

6.  Be sure to have surface signalling aids with you, such as a Dive
Alert,
and delayed SMB. The Panga (what they call their inflatables/RIBs)
may
not be within shouting distance when you surface. An ordinary
whistle is
next to useless in windy conditions/surface swell.

7.  If you are on a liveaboard (recommended) you would need to be
mentally
self-sufficient where the diving is concerned. In all probability
you
will be diving as part of a group, but in actual fact you will have
no
dive buddy. A "solo" mentality is healthy with this type of diving.

8.  Go! If you feel cold on the dive you can surface, if you don't like
the
current, surface. Half a dive in the Galapagos is worth two
warmwater
dives on a mediocre reef. You will not regret it, and the memories
will
stay with you forever! The overall experience of diving in nature's
wonderland will enhance your diving prowess no end - the life, the
environment, the variety. Topside excursions are just a bonus.

BTW we are in the same age group. Also, if you are still at the
research stage, look up Quasar Nautica on Google. It may save you a few
dollars/pounds. I used them, but booked through an American operator.
Was on the "Mistral" boat, which was very good when compared to a dozen
others I've seen. Just to qualify my opinions/advice, I am a coldwater
(mainly) diver/
instructor, 1,400+ dives, dive 12 months of the year.
Hope I have been of some help.

Enjoy!

Seadeuce