If you think that the only use for a trisuit is to keep you warm in a cold open water swim, you are wrong. It creates many more advantages than just keeping you warm and adds to the swimming phase of the triathlon. The advantages far outweigh the disadvantages.
The most obvious advantage, the one that you probably thought of first, is the fact that trisuits keep you warm in open water swims. The reason for this is that you are adding 3-5mm of neoprene material to your body. Besides keeping you warm, it allows you to move more efficiently through the water. The outside of a trisuit is designed to reduce drag and increase speed with little effort. This type of material has closed cells with helps you swim on the surface of the water.
To get the right type of trisuit for you, you cannot simply grab it off the rack. If you attempt to use just a wets that you borrowed from a friend you are going to be in for a surprise. Diving and surfing suits are built to be used differently and will work against you in a triathlon. The thickness that these types of suits allow for minimum flexibility. A trisuit that is fitted correctly should have excellent flexibility around the arms and shoulders. If you do not have enough room in these areas you will be fighting against yourself. Although trisuits are thinner when compared to diving and surf suits, you still need to find a material and thickness that will work for you. Some suits have areas that are thinner than other parts of the suit and this may contribute to heat loss.
When you are picking out a trisuit be sure to find one that fits well. If you do not find one that fits properly you will be uncomfortable and it will create additional drag that you do not need in your triathlon. You also need to use some sort of lubricant on your skin to prevent chaffing such as Body Glide. You will be happy you did later. Be sure to also make sure that the zippers and seals are working properly. You want to do this so you can be sure that you are able to get out of your suit later after the swim portion of the race.
Just remember that race day conditions will dictate whether or not you will wear a trisuit. The water temperature needs to be below 78F in order to wear a trisuit. If you chose to wear it at a temperature over 78F, the time that you get will not count. In order to insure the best results that you are capable of, focus on your training and don’t depend on the trisuit for buoyancy and speed, although it helps!