Okay, there is a new term for wetsuits, "open cell." Open cell used to mean neoprene that was not "closed cell," in other words, had no buoyancy because it absorbed water as the cells were open to the water. I found out just last year that "open cell," is now a term used to describe what we used to call "skin one side," or "skin both sides." In other words, a suit made of closed cell foam neoprene without any backing, or with backing on only one side. We loved the "skin one siide" suits because that meant that the neoprene was glued together to be watertight. The stitching was only through the top, nylon or other material, layer, and the joint was completely glued together and therefore watertight.
It is thin watertight seals that makes a "wetsuit" really warm, as it prevents water from circulating through the suit through the stitched seams. Most suits today have material on both sides of the suit, and don't use "blind stitching" to stitch together the seams. That makes them leak water. In cold water (like freezing water) that makes for a very cold suit.
That terminology for the suit has changed, and "open cell" is no longer what I described above. Open cell suits are suits with the skin of the neoprene and the glued seams that are watertight.
So, with an open cell suit, which is the warmest? The warmest is skin-in, and the nylon/clothe on the outside, with no zippers. We used to call these "pull-over" suits with an attached hood on the jacket. "Farmer John" pants, which cover the chest area, were under the pull-over jacket.
Well, the next warmest is a jacket with only a front zipper, and none on the arms (or an outer zipper with an inner cuff that's waterproof). The "Farmer John" pants were worn under that, and if it's really cold water, a hooded vest under the Farmer John pants. If the water is not frigid, then a good hood with enough material to tuck under the jacket is good too.
The hood needs special attention, and should extend the face opening down to the eyebrows. Then the mask in worn with the strap over the hood, but the skirt under the face opening of the hood. Many hoods are not made this way, and have a large open area on the forehead with the forehead skin exposed to the water, the COLD water; this makes for instant headaches in extremely cold water. The chin cup needs to protect the majority of the chin, with only the lips showing for maximum protection from cold. The mouth can still hold the regulator, but then the cold water exposure to skin is minimized.
Boots should also have waterproof seams, and even ones with zippers now have this. But be sure that the seams are waterproof, as water coming through the boot zippers can make for really hold feet.
The same is true for diving gloves; make sure that they are waterproof (no water through zippers). We used to hand-make our dive gloves, which were actually 3-finger mitts. Separate the thumb and forefinger, then keep the other three fingers together and on a sheet of paper draw an outline that is about 1/2 inch from the skin. That is one side of the 3-finger mitt. Turn over the outline, and cut out another piece, and glue them together with neoprene cement. To do the gluing, first coat the surface with a coating of neoprene cement, and let it dry completely. Then, give it a second coat, and wait a few minutes until it is tacky, and join the edges together. This makes a waterproof seal, and these gloves can be used in extremely cold water. We used to use 1/8 inch neoprene for our gloves, but now they are in millimeters, so use 3 mm neoprene for maximum dexterity, or 5-7 mm for maximum protection from the cold water.
If you gear up for cold water diving with these suggestions, you can comfortably dive in almost freezing water for about an hour, up to say 60 feet (20 meters) depth. Beyond that, with wetsuit compression, you probably will want a dry suit.
SeaRat