Below is taken from an article by Harry Needs, Olympic GB Swimmer – http://www.theguardian.com/lifeandstyle/the-swimming-blog/2013/sep/12/open-water-swimming-tips-harry-needs

Start small

Enter smaller open water competitions and then build up from there.

Choose your stroke and stick to it

Freestyle is the strongest, most endurable stroke. People can do other strokes for a long period of time, but when you look at even pool swimming, all the long distance events tend to be freestyle. So keeping a constant stroke rate and kick across the board is key. But beware … open-water events are often 5km or 10km races, so like a marathon runner, you don’t want to start off too fast.

Arm action

In open water, it’s not as important to have a high elbow; in fact, because the waves can be high, it’s more about swinging the arms over the waves and then grabbing on to the water.

Try a two-beat kick

In pool freestyle, people normally do a six-beat kick – so it’s a constant kick against the arm stroke – whereas in open water, a two-beat kick is usually better as it helps preserve energy. So it’s just a flick of each leg, every two strokes.

Decide on your breathing pattern
A lot of people tend to breathe every two freestyle strokes in the pool, or if they’re sprinting they might hold their breath throughout a whole race. But they also tend to then get really tight on one side, which is why I’d encourage bilateral breathing wherever possible. In distance swimming, you’ll most likely breathe every three, and then when you’re really tired you can go to every two. Also, some people do a water polo stroke, so they can lift their chin above the water, have a cheeky look at where they are and then breathe and go back down. People who swim open water tend to over-rotate, so they can get a clear vision of where they are (whereas in pool swimming, you’ve only got to turn your head a little bit just to breathe).

Be aware of your surroundings
In open-water races (a triathlon, for example) you’re usually swimming in a pack and there’s no lane space, so you have to be very wary of surroundings. People do literally punch you, scratch you … and yeah, that’s sometimes on purpose (maybe because it can’t be seen underwater).

Weather conditions
Open-water swimming is very dependent on weather conditions. If it’s windy or the current is strong, it’s a bit like golf, where people throw the ball in the air to see what the wind direction and strength is like, and then hit it slightly in the opposite direction to the wind. You’ll need to swim into the current on a diagonal to counteract it.

What’s in the water?
People come out with stings sometimes, if there are pools of jellyfish. The best way to avoid that is to wear a wetsuit.

Kit
You’ll need a wetsuit if it’s really cold (especially skinnier people as they can get hypothermia without). This also makes you more buoyant by trapping a layer of air between the skin and the wetsuit, allowing you to stay higher above the water. If you’re tackling waves, you want to be as high as possible.

Eyewear is very individual: some people like to wear bigger goggles so they can see more of what’s around them and see where the people are and if they’re heading in the right direction. Reflective goggles are key because there can be so much sun and you don’t want to be squinting all the time.

Rest and food during open-water swims
What if you need a rest in the middle of an open-water swim? You could tread water, or just skull above the water to collect all your thoughts and prepare yourself for the next part. In more serious, longer open-water swims there are feeding stations where people can have a bite of banana or an energy bar – or a sip of water, just to clear all the salt and gunk out of your mouth. In those swims you need to snack, but it’s very hard because you have to arrange with the people on your team to tie the food on to the end of a stick (because you’re not allowed to have any physical interaction with people), so as you come into the feeding station you have to look up for your team to get that bite of banana in. Missing that feeding station will massively affect the end of your race, so it needs to be sorted out.

What about supplementary land-based training?
Any endurance training will help – long runs, for example, or long cycle rides. Stay away from weights – many distance swimmers tend to do that anyway, and go into more bodyweight circuit-based work, where you’re doing an exercise that involves some power and strength but you hold it for longer, like a boxing circuit.