WETSUITS

The universal material used for thermally insulated wetsuits is foam neoprene. It is the gas in the bubbles that provides the insulation.
Unfortunately it also introduces the problem of reducing buoyancy with depth as the material loses volume under pressure. An additional effect of reduced volume is reduced insulation, so the deeper you go the colder you get.
Wetsuits are not the first choice for deeper coldwater diving, but work rather well at more modest levels. There is an almost direct relationship between the thickness of neoprene needed and the anticipated temperature range. Two to 3mm is fine in the tropics, 5-6mm in the Mediterranean and 7-8mm in those Northern climes.
But material thickness alone is not the answer. If you can keep almost all the water out of the suit you will stay warmer. This is done first by getting a very good fit and then by reducing flow-through to the minimum with smoothskin seals at neck, wrists, ankles and possibly the face.
A special zip-backing is also desirable. Such "semi-dry" suits are comfortable to wear, and surprisingly warm. Price aside, there is little justification for building a wetsuit today which makes no effort to reduce flow-through.
Recent years have seen great improvement in the design of one-piece suits, combining panels of different thickness in different parts of the suit to give high insulation where it is most needed, and greater flexibility around the joints.
The one-piece has fewer coldwater entry points, so the problem is attacked at source.
It is argued that with the two-piece suit you have a double layer of material around the trunk, keeping it warmer. However, this is the one part of your body that changes dimensions with every breath you take, so two layers of 7mm neoprene have a negative effect on your ability to breathe freely, and can cause "out-of-breath" problems.
Warmwater suits need not be thick, so have superior stretch qualities. A 2-3mm suit will stretch to fit to a reasonable degree, but do not skimp on the quality of the outside lining, as you still need protection from abrasion and things that sting.
The travelling diver may find a layering system the best solution to being well-dressed on all occasions. A system such as Scubapro's Hi Tech Steamer offers a shorty vest, a one-piece steamer with back zip and a sleeveless short-legged jacket with front zip as a top layer.
Just choose the combination of layers you need for each dive.