I remember my last wetsuit dive.
It was December 6th, 2003 - and we were at Willis Point. I was renting gear to get a dive in; this was before I owned all my kit. It was the standard wetsuit - 7mm farmer, 7mm jacket, 5mm boots / gloves / hood.
In the Saanich Inlet, there's a thermocline at about 40 feet. The temperature in the summer drops to about 4C. This makes for great vis. I've seen 120 feet in the summer. In the winter; it's colder.
I was the only one there in a wetsuit. In retrospect, this should have been a good clue. Nevertheless, I had packed warm clothes and hot cocoa, so I'd live.
At this point, I'd like to mention that my log book is at home with my dive gear. When I said "I remember my last wetsuit dive.", I meant that I remember my last wetsuit dive.
My second clue (warning?) was when the divemaster went over signals. The first one was "cold". He went over it with me three times. I don't remember going over any other signals.
All my dives are in the Pacific, up north of what you'd call the Pacific Northwest. My OW checkout dives were at McKenzie Bight. I'm mentioning this so you don't think, "well, yeah, if you learned on vacation, I'm sure it felt cold." Not so! -- my last vacation was my honeymoon 10 years ago, long before I'd picked up my first reg.
We got into the water, and it was indeed cold, but not much colder than the normal dives I'd been on. I did my usual bracing, and settled in after the initial shock. The thing about the ocean here is that it's roughly the same temperature all year round.
Surface temperature.
After descending below the thermocline, I felt cold.
When I say cold, I've been swimming in the open ocean in swim trunks. I've been on polar bear swims in fresh water lakes with snow on the ground. I've swam in glacier-fed pools in a mountainous river.
So, with those qualifications, I mean cold. Right now I have to go into a little thermodynamics. Any exposure suit, terrestrial or aquatic, doesn't keep you warm. What it does is provide a barrier to heat transfer. In other words, it slows down the rate where the heat from your body will dissipate into the surroundings. In the long term, all hot bodies will eventually reach the temperature of the surroundings. A wetsuit transfers heat faster than a drysuit. An air-filled drysuit will transfer heat faster than an Argon-filled one. (And yes, some people on this dive were using Argon.) Usually, you can tolerate the heat transfer for a limited time. If this heat transfer is more than you can tolerate, then you get cold. In the extreme case, you die. In all cases, if you are cold, you will consume more air.
We went and saw the sights at Willis. It's a nice dive site, and I'd recommend going if you get the chance. We saw stout lobster, Spanish dancers, and a myriad of other critters. No octopus sightings, but we saw a den.
I remember the safety stop clearly: we're all at 15 feet, looking around at the shallower lifeforms. (The advantage to a shore dive is that there's always stuff to look at.) I remember the divemaster pointing at me and asking "cold?". I replied with the pre-arranged "cold, but okay" sign. I thought about kicking back, opening the suit, and fanning myself off.
After a few minutes, we swam up to the surface and walked to our cars to change.
It's December 6th, and it's started to rain. The ground was muddy and cold. It was here where I started to get fantastically cold. I was miserable, wet, chilled to the core from the dive, and now my extremities were being wind-blown and rained on, with a cold, nearly-freezing rain.
I was still wet, and trying desperately to towel off while keeping from, ahem, flapping around. With the cold, I might have left, shall we say, the wrong impression. Also, there are many houses around the area, and they already have a low opinion of divers.
After finally getting into warm woolen socks (A sensation I still remember five years later) and having a hot cup of cocoa, I drove back to the dive shop, vowing to get a drysuit.
I bought a used one from a co-worker. I've been diving dry ever since. My first was an old Gates shell suit. I now dive in a 3mm neoprene drysuit. Most of the year, I can wear just a pair of spandex pants / dryfit shirt underneath. For the winter, I throw on a fleece shirt.
If you're going to dive in cold conditions, get a drysuit. End of story. Even the fanciest of fancy wetsuit is going to be cold.
The dive I did at 29* was practically a bounce dive because one of the other people on the dive had a reg freeze and freeflow within a minute of hitting depth (about 115ffw). 29F was the coldest, at depth. The water above the first thermocline was about 50F, once we got shallower the reg quit freeflowing and we finished the dive swimming around at about 30ffw and about 42F, with a total dive time around 25 minutes if I remember correctly, I'd have to pull my logbook out for extact details..It was COLD though.
It was a rental from the shop, so I'll guess scubapro? There were only three of us who dove wet, and that was the last time any of us got in the water for the day, we took the boat back to the dock and started a fire and let everyone smart enough to own a drysuit finish out the other two dives. Haven't been too deep in any of the lakes back home since 
