Not real sure why this is so confusing. 'billt4sf' pretty well nailed it in his comments post 26. To reiterate, heat is transferred by conduction, convection and radiation. Radiation can be discounted as negligible since it mainly occurs at high temperature. You can feel the heat transferred through radiation by standing in front of an electric radiator heater or the coals of a fire.
If you hold a steel bar at one end and heat up the other, eventually the heat will travel through the steel bar to your hand providing the bar is not too long. That is conduction. If you wind the windows down when starting off in a car that has been sitting in the hot sun the hot air gets blown out and replaced with cooler outside air. That is convection. If the bar in the heat transfer by conduction example was long enough then you'd find that a lot of the heat would be dissipated into the air by convection and there would be little if any temperature increase at the hand holding the bar.
Now how does this apply to the various dive suits? Your body is typically at a higher temperature than the water so heat flows from your body to the water.
A wetsuit minimises heat transfer by conduction and convection. It provides at least some insulation. If you want to be technical, the wetsuit has a lower conductive heat transfer coefficient K (W/mK) than the boundary layer would provide at the skin/water interface. In addition the wetsuit serves to minimise the water movement at the skin/water boundary layer and so reduces heat transfer due to convection. Some water will still flush through the wetsuit which means that there will still be a significant amount of heat transfer through convection. That cold blast of water you feel down your back is the cold water is convection ie. cold water displacing warmer water. Not a bad thing for those who pee in their wetsuits.
Some people wear an extra layer under their wetsuit to stay warmer. That will reduce the heat lost through conduction but if you still have water flushing through the suit it probably won't make as much difference as you would hope.
Enter the semi dry. I've got a 6.5 mm Nova Scotia semi dry and turn the ends over at the ankles, neck and wrists to provide a seal at these locations. The zip is also a better seal than a typical wetsuit zip and is aligned horizontally across the back which reduces the amount of water flowing in past the neck seal. I used mine on the weekend in water that was 19 degrees C. As I took the suit off after a dive about half to one litre of water poured out of the suit. You can feel the water is warm water and no I didn't pee in the suit! It gets heated up because it is trapped and close to your body. The heat transfer through conduction won't be a lot different to a wetsuit the same thickness but the convective heat transfer will be a lot less because there is less flushing of water through the suit.
A dry suit is potentially the warmest which is why they are worn in frigid conditions. There is no heat transfer through convection if the various seals hold. I've never worn a dry suit. An air gap will certainly reduce the heat transfer through conduction but my guess is that the suit would to some extent cling to the diver and at those locations the air gap would be minimal. Wearing an undergarment would help to maintain the air gap and hence improve the insulation properties of the suit and minimise heat transfer through conduction.
My guess is that most of the heat loss is through convection ie the action of water flushing through a suit but I'm still uncertain on this one. The deeper you dive the more the wetsuit is compressed. This will reduce the suits insulation properties and increase the heat transfer through conduction. I'd probably be cold in water at 18 degrees C at a depth of 25 m in a 2 mm semi-dry (if such a thing existed). A 2 mm dry suit is not uncommon and I'd be interested to hear if divers find this adequate or would need an undergarment in these temperature. It seems likely that the material in a dry suit provides better insulation than a semi dry suit. It seems to me that water can permeate through a semi dry suit which would explain the better insulation of a dry suit but I'd need to have that confirmed.
On the question of the relatively high heat capacity of water, that would only be a benefit in a semi dry suit. My experience is that the dive suit reaches thermal equilibrium fairly quickly. If the heat loss through conduction is relatively small then increasing the reservoir of water trapped inside the suit won't provide a lot of extra warmth. It wouldn't be hard to let in a bit of extra water into the suit and see if it this makes any difference. You'd be colder for a lot longer at the start as the water inside the suit heated up. I've never seen any discussion that doing so provides any significant benefit.
---------- Post added August 25th, 2013 at 11:36 PM ----------
The short answer is as follows:
Heat is lost through the water that flows through a wet suit or semi dry suit. The less water flowing through the suit the warmer it will be. That is why a semi dry is warmer than a wetsuit. Water is not a good insulator. Air is. That is why dry suits are warmer than wetsuits or semi-drysuits.
I always let some water in. Allows the suit to move into a comfortable pisition.
