Open water for the first time is sometimes an unexpected results. A really efficient swimmer who is very strong has never had a suit that restricts lung expansion, add to that cold water where the lungs are again struggling for air, now add warm up with a high heart rate and a large need of oxygen and the struggle begins. It can be wheezing, it can be coughing up stuff, it can be clostrophbic reaction to the tight feeling and the lack of vision in the water, whatever it is, it is a reason to slowly get introduced to the water in a different way with less expectation to swim.
Start with everyone getting into their wetsuit, group people into small groups with floating frisbies, start on land to get a small bit of fun and some on land warm up going, get the air going into the lungs without the introduction of cold water and lack of sight. Once the game is underway begin launching the frisbie more and more into the water, each person is wet to the legs and still actively warming up as they introduce themselves to the coldwater, as the frisbie is starting to make its way out into the water, you have more expectation to swim to retrieve it, short distance throws might be head up swim, further throws might be face in swim with sighting. All of this is pertinent to the sport and it a way to divert the attention of the athlete to the game and not into the nerves and once properly warmed up and getting used to the restriction of their suit on land, they are less likely to feel it so constricting once entering the water.
Each person has small amounts of swim for retrieving and are now floating in between where they cool down and get used to being in the water. At this point you can bring everyone into the shore and begin a small group race. Have one person swim out to be a bouy, when they are 100 feet from shore, set your athletes up and let them know what direction to swim in around the bouy from left to right etc so there are no collisions.
After 3 of these are run, get a new volunteer for a bouy, have that person actively move to throw off the swimmers, make them more focused on sighting and manuevering. They have done small bits of swim, they have had some contact in a rather close to shore short burst of adrenaline, once everyone has been out a number of times, it might be a good time to start an out and back swim. New swimmers may still succumb to cold water reaction that cause wheezing and coughing, make sure you are swimming along the shore, not directly out from shore, this allows for warmer water and less nerves for the swimmers.
I have also used inflatable dingies and pulled them with surgical tubing around my shoulders. THis is a great way for people on shore to know where the group is swimming as well if someone has issues they have a floation device to hang onto that is close by.
We are not yet using kayaks, these would be very helpful for assisting nervous swimmers as well.
Basically if you put a little fun in to divert the new triathletes attention to a game they are a little less likely to go into the water cold, with nothing but body awareness starting to send them nerve wracking messages.
Have fun in the open water, no lines to follow only freedom.