I currently dive in a 7 mm Neoprene wet suit. It is of the Farmer-John and jacket variety, but a diagonal zipper rather than a 'beaver tail' as  I can then wear the top by itself in warmer water. I also wear, when needed, 7 mm gloves and hood. A picture of me fully kitted up is available at the end of this page.

The change in water temperatures requires a constant change in thermal protection if you want to maximize your diving comfort. Wearing a lot of Neoprene requires wearing a lot of lead to compensate for it's buoyancy. However, the Neoprene compresses at depth; it has lost roughly half it's buoyancy at 33 feet and three quarters of it's buoyancy at 100 feet. Thus at 100 feet, the chances are you will be 15 lbs overweight. This must be compensated with air in the BCD. The less Neoprene I wear, the less weight I need to carry and the easier buoyancy adjustment. Wearing no Neoprene, as when in Cozumel, say, then buoyancy control is just a matter of weightless swimming up or down as desired or simply breathing.

This is why, throughout the season, as the temperature varies, I discard as much Neoprene as possible. The following table provides the temperatures at which I wear different gear. Bear in mind that I weigh 170 lbs and am fairly fit; also, this is fresh water. For those from salt water who may be diving in fresh water for the first time, start by dropping 6-8 lbs of lead. Note that this table is just a rough indication of my diving activity. If I'm performing multiple dives a day, then I may dress more.
 
 
 

Temperature Thermal gear Weight 
Comments
32 F - 40 F
Full suit, socks under booties
20 lbs
15 minutes maximum bottom time and cold induced cramp is a danger
40 F- 45 F
Full suit no socks
20 lbs
30 minutes maximum bottom time
45 F- 55 F
Full suit no socks
20 lbs
No bottom time limit.
Multiple dives 
55 F - 60 F
No Neoprene gloves just light gardening gloves for protection
20 lbs
At last, I can feel & manipulate things
60 F - 65 F
No hood or gloves
20 lbs
The freedom to move the head is great
65 F - 70 F
Drop Farmer John, now just the 7 mm shorty
10 lbs
Less weight. Buoyancy control is now much easier
70 F - 75 F
3 mm shorty
4 lbs 10 lbs in
salt water
 
Above 75 F
Naked, actually shorts and T-shirt
0 lbs 8 lbs in
salt water
Bliss. However, if I run my tank down to 500 psi in salt water, I need 8 lbs at 15 foot.

How this affects my buoyancy can be seen from the table of buoyancy and weight of me and my equipment.
 
 

Equipment item Dry weight
lbs
Minimum buoyancy
lbs
Maximum buoyancy
lbs
Your's truly 5'9"
165.0
-2.0
+2.0
7mm hood & gloves
1.0 
0.5 @ 100 ft 
+2.0 
7mm Farmer John
4.0
2.0 @ 100 ft
+8.0
7mm Shorty
4.0
2.0 @ 100 ft
+8.0
20lb weight belt 
20.0
-17 
 -17
AL80 tank
40 full
-4.0
+1.0
Twin steel 72's plus manifold 
73 full
-15.0
-7.0
Cannister light
9.0
-3.0
-3.0
Octopus
3.0
0
0
Regs
3.0
0
0
BCD
5.0
0
+45.0
Steel backplate & harness
7.0
-5.0
-5.0
Steel backplate & harness & single tank adaptor
9.0
-6.0
-6.0
Reel & Pelican 8D light
8.0
0
0
Steel 72 sling bottle with reg
41.0
-6.5
-2.5
Twin steel 85's plus manifold 
82 full
-20.0
-9.0
Backplate weight
7.0
-6.0
-6.0
Twin Pressed Steel 104s plus manifold
121 full
-25.0
-11.0

The numbers in this table are only approximate and assume a density of air of 0.08 lbs. per cubic foot and tanks with 20% of gas when "empty". I give the change in my buoyancy as +/- 2lbs which is reasonable with regular breathing for an adult male. I haven't measured this yet. The wet suit compression also is only estimated; it will be measured this season. The following chart shows how buoyancy changes throughout a dive wearing a full 7mm wet suit.

This photo is me in full wet suit gear taken April 1997 during my Rescue course. I weigh 170 lbs and my gear weighs 80 lbs. Thus I must displace 250 lbs to achieve neutral buoyancy. In salt water, I require 250 x 0.026 = 6.5 additional pounds of negative buoyancy over fresh water, as salt water is 1.026 times the density of fresh water. An extra 8 lbs of lead on the weight belt is required.
 


 

 BREATHING AND BUOYANCY


This diagram, taken from the 1970 US Navy Dive Manual, represents lung volume over a rest/work/rest cycle. Note that a volume of 1 litre is approximately 1 quart, and represents 2.2lbs of fresh water. The total lung capacity is self explanatory; the vital capacity is the maximum volume of air that can be expelled from the lungs after a full inspiration; the residual volume is the amount of air that remains in the lungs after a full exhalation. The inspiratory and expiratory reserve volumes represent  the additional volumes available for increased breathing. The given tidal volume is that for a man at rest; in water this would create a 1 lb change in buoyancy over the respiratory cycle; the maximum buoyancy change possible is when breathing involves the complete vital capacity of the lungs, which represents a buoyancy change of around 10lbs. If you're breathing this hard under water, I suggest you take a rest.

What is interesting about this diagram is we can see how to control buoyancy through breathing. Suppose we are in no current and relaxed, we could expect a tidal volume of about 1 litre, equivelant to a buoyancy change of about 2 lbs. We would be breathing this with about 1 litre of air always in the lungs - the expiratory reserve volume. If on one breath, we were to breath in a volume of 2 litres of air and then returned to our 1 litre tidal volume breathing, we would have increased our expiratory reserve volume to 2 litres, thus we have increased our buoyancy by an additional 2 lbs.