Ed points out that "the correlation between swim performance and overall race time is poor, which means your swim time does not have a big impact on your placing." The event is simply too short to have a material impact on the race. But it can make all the difference if you are vying for one of those coveted Hawaii spots. I am a poor swimmer and I regularly finish in the last 1/3 of the swim. Since I often finish in the roll-down spots, my slow swims have definitely kept me from going to the Big Island. Therefore, I fully understand the desire to "do something about it." Five minutes would make all the difference for me.
What further confuses people is all the attention paid to technique. Technique implies something you can learn to master. There is no doubt that technique makes a big difference in Olympic swimming events, esp. when reaching for the wall touch. But such things matter little in long open water swims.
Humans are incredibly slow swimmers and water is a really dense medium. Any small amount of excess resistance can really slow you down. What we really are talking about here is slow and slower so efficiency is key.
For all the talk about Michael Phelps setting "a blistering pace," and "sprinting to gold," few people realize that the average adult could out-walk him without too much effort. And that is for short events. The "long swims" in this category are about 1/4 mile. For any real distance, a "speed" of 2.75 mph is considered world class.
Over the past five years I have watched a lot of swimmers, and learned a lot about swimming. Here are some things I learned.
1. Body type is the most important attribute of a swimmer. It is something you are born with and there isn't much you can do about it. It helps if you are tall, have a long torso with relatively short legs, big feet, a small head, and a wide arm-span.
2. Lie. Body type is the number one determinant of how you lie in the water. The flatter and higher you are the better but despite all the recommendations about keeping your head down, and pushing on your chest, etc. , your build is key. Some people stay flat even when their head is up, while others hang down despite their best efforts.
3. Muscle. Swimmers benefit the most from weight lifting. The more muscle you can pack on your upper body the faster you will swim. There is little or no penalty for the excess weight -many swimmers are overweight by athletic endurance standards- and more muscle means more power. The upper body is less subjected to gravity so "specialized training" such as weight lifting helps more here. Some swimmers look like weight lifters or wrestlers.
4. Body sense. People with better body sense, i.e. people who "instinctively" know where their body is in space tend to do better. But it is hard to see what is cause and what is effect. Good dancers make better swimmers.
Corollary: if you can get objective feedback (from a mirror or video) it can help a lot. Observers are not nearly as good. Many can tell you that you are doing something wrong, but very few can put their finger on it, or tell you how to correct it.
Some triathlon/open water guidelines:
5. Wetsuits. Wetsuits help but not all that much. They help the better swimmers more than the poor swimmers. You can gain about 5 minutes on an iron-distance leg. You can lose almost as much if you can't get out of the wetsuit. Still, better to wear one to keep warm unless the water is tropical.
6. Draft. Drafting is legal in (triathlon) swimming and it helps a lot.
7. Stay calm. Many people panic in a mass start. Then they run out of breath.
8. Swim straight and swim the shortest line. This is the area where the most gains are possible. It is also what fools most people into believing they can swim faster. As they become more comfortable, athletes start closer to the best line and further ahead (where it is more crowded), hence they swim shorter, straighter and get a better draft.
Everything about swimming is slow so it should not surprise you that swimming is a sport where making progress is agonizingly slow too. The rule is that once you can easily swim an iron-distance, you will also know your best swim time for the event. I.e. your gains after that will be minimal, even if you spend inordinate amounts of time in the pool, or spend inordinate amounts of money on coaching. As a matter of fact most of your gains later on will come from unrelated sources. Better draft, less current, less wind, straighter swim, shorter course (yes it happens, the opposite happens too believe me!), etc. all matter a lot more than your actual swim prowess.