You should use a well-fitting, one-piece neoprene wetsuit, which ideally is specifically designed for swimming rather than other activities such as diving or surfing. Swimming-specific wetsuits are made from neoprene panels of varying thickness to provide buoyancy at the same time as allowing more mobility where required e.g. around the shoulders.
The most important thing is that whatever wetsuit you decide to wear –, shortie, short arms, and legs – it fits well. It should be snug when you put it on, so the water trapped next to your skin is warmed by your body and provides insulation.
We recommend that you try the wetsuit out before event day to ensure that it is comfortable to swim in. If the wetsuit is loose or ‘baggy’ or if it is too tight around the neck and you feel that you have to unzip it to breathe more comfortably, water will flow through it compromising insulation and potentially restricting your swimming.
For the avoidance of doubt, triathlon suits made of lycra (or other similar synthetic materials) do not offer buoyancy or insulation and are not accepted as wetsuits at Great Swims.