Drysuit vs. Wetsuit

Water conducts heat away from the body 20 to 25 times quicker than air so it is common in SCUBA diving for the diver to wear an exposure suit to protect them from losing too much heat too quickly. There are two main exposure suits that insulate the body while diving; drysuits and wetsuits. Drysuits work by sealing a layer of air around the body (thereby keeping the diver dry) whereas a wetsuit works by trapping water next to the body.  Drysuits will keep you warmer for longer and are necessary for cold water diving, however they lack the flexibility and comfort of a wetsuit. Wetsuits are only suitable for warmer water diving and allow the wearer more freedom of movement in the water. Wetsuits generally cost between 10GBP to 300GBP whereas drysuits are a more complex design and can cost 400GBP to 2000GBP.
The wetsuit
Wetsuits mainly vary on their thermal protection based on how much of the diver is covered in neoprene- a type of synthetic rubber used to make wetsuits- and the thickness of the suit.Shorty-wetsuit
The two main sizes are full wetsuits that cover the whole body, and “shorties” that stop at the knee and elbow. You can also wear additional layers over a wetsuit to add extra insulation and these are sometimes referred to as “pieces”.    
There are also semidry suits which act more like a wetsuit but have tighter seals on the wrists, ankles and neck, and are generally thicker than regular wetsuits. For the rest of this article semidry suits will be included in the classification of wetsuit.  
The drysuit
Drysuits cover a diver’s whole body except the hands and head. They are made from a waterproof material and have bands of latex or neoprene at the wrists and neck that form watertight seals. This keeps a trapped layer of air within the suit that insulates the diver from the water they are diving in and keeps them dry. The suit needs to fit well and the seals need to be tight to ensure the suit does not allow water in.  
There are two types of drysuit; those made of membrane and those made of neoprene. The material used to make membrane suits is made from layers of nylon and rubber, whereas neoprene suits are made from the same material as wetsuits. Both types have watertight seals around the neck and the wrists and the diver enters the suit through a water-proofed zipped opening. The type of drysuit a diver chooses to use is down to personal preference, it is not uncommon to hear the arguments for and against each type amongst diving circles- each have their pros and cons.
Which to choose and when
Assessing the conditions you expect to encounter at your given dive site is important to decide which type of suit to wear. The colder the water the more likely you will need to use a drysuit so finding out the water temperature at the given time of year is advisable. The duration of the dive is another important consideration, for example technical divers diving for a long period of time even in a warmer climate may decide to use a drysuit when completing dives that potentially last hours. Also consider the expected weather conditions and temperature on the surface; a drysuit will protect you from windchill far more effectively than a wetsuit can.  If you have not dived a site before it is a good idea to contact the dive centre nearest to your dive site for advice on what to wear.

However whether a diver decides to use a drysuit or wetsuit for a dive depends a lot on the person using it. Some people are more comfortable wearing thicker wetsuits and do not feel the cold as much as others however someone else may prefer to use a drysuit for a wide range of temperatures. 
In-water use
When wearing a wetsuit, as you dive deeper the neoprene compresses and the suit becomes less buoyant and thus you need to add more air in your BCD (Buoyancy Control, or Compensating, Device) to compensate for this. The compression of the material also means the suit is now thinner and will not be as effective an insulator. So on deeper dives you will need to use more air to stay neutrally buoyant and your suit will not keep you as warm.
Using a drysuit requires additional training, as it acts as a second buoyancy device, alongside a BCD. Drysuits function in a similar way to a BCD in that you can fill them with air by using a low pressure inflator valve, usually located on the chest of the suit, and a dump value, either located on the shoulder or the wrist.  The amount of air in the suit can thus be controlled by a diver. This is necessary because as a diver starts to descend the air in the drysuit begins to be compressed and can cause a squeeze on the diver's body. The compression of the air can be counterbalanced by adding air to the suit and this is also used to maintain neutral buoyancy. When ascending the air expands in the suit and needs to be released from the dump valve, similar to using a BCD.  
Training to use a drysuit will include methods of in-water positioning to reduce the risk of a feet-first/inverted ascent, which is caused by air becoming trapped in the drysuit boots. It is advisable to practice these skills after training to become fully confident in using a drysuit.  
There are a wide range of conditions where drysuits are used to protect a diver against the cold. The most common reason is for use in colder sea temperatures, for example temperate water such as off the British Isles, where some divers choose to wear drysuits all year whereas other divers change to wetsuits in the summer months.  For ice-diving where a diver is subjected to extremely cold water a drysuit is essential, and drysuits are also commonly used in cave diving as many caves have cooler water and the dives can be much longer in duration than recreational dives. As mentioned before technical divers may use drysuits in warmer waters where they dive for prolonged periods, but may also use a drysuit as their mandatory back-up buoyancy device. 
Really the choice of when and where you use a drysuit or a wetsuit is down to the individual but it is worth taking the advice of locals and fellow divers to help make up your mind. What is right for one diver is not always right for another and it is up to the individual to decide what feels best for them. 

Text by Neil Watson and Ellie Taylor 
Photos by 
BoaNoite and dive90m