What the expert says ...
Julie Bradshaw MBE is one of the world's top open-water swimmers. She holds 15 world distance records including the fastest butterfly crossing of the Channel.
Increase your range
Sea swimming isn't necessarily all about long distances, but it's easier to cover the ground when you're in the sea as you don't have to turn around after every few metres and there isn't (usually) anything to bump into. It's worth building up your distances in the pool over the winter if you intend to swim a long way in the sea, and you can improve your general fitness with a bit of running and rowing, or other aerobic and endurance work.
Whether you're swimming a few hundred yards or several miles, have a positive attitude and be realistic about what you want to do. Getting into a rhythm can make time appear to drift by - I've known an hour seem like five minutes on some of my really long swims. A long sea swim can be almost therapeutic, mentally as well as physically rewarding.
It doesn't matter if you're not super-lean. A bit of body fat can be a distinct advantage in cold waters, in fact, as it provides some insulation.
Regular swimming will build muscle strength and improve cardiovascular fitness and endurance, and can be used as an excellent form of cross-training. Mastering the elements while sea swimming can build confidence and self-esteem, and it's recognised as being an ideal way to assist recovery from other sporting injuries (injuries from swimming are very rare).
Watch the cold
Hypothermia can be a risk. Listen to your body and if you start to feel cold, get out of the water. You also need to be wary of sunburn and use a high-factor waterproof sunscreen on areas such as neck, shoulders and the backs of your legs. Dehydration is another consideration, particularly in sunny and/or warm conditions - you may be in the water, but you still need to drink regularly. For longer swims you may need to apply Vaseline to areas of possible chafing, especially if you're wearing a wetsuit. Even without a wetsuit, you should apply Vaseline around the edges of your swimming costume, under your arms, at the bottom of your swimming cap, etc.
Most people graduate to sea swimming from the pool, so it's important to be aware of the differences between the controlled environment of a pool and the natural environment of the sea. The lack of guidelines or lane ropes to keep you on course, and the fact that weather and swimming conditions can change in minutes, can take time to adjust to.
A swimming/triathlon wetsuit will set you back some $$$. Good swimming goggles (try Speedo or Zoggs) are vital, as are earplugs to help prevent 'swimmer's ear', a bony growth in the ear canal. You will also need a waterproof sunscreen, a bright swimming cap, so you can be seen by other water users, and a jar of Vaseline just in case.
Fear of the unknown
Even small fish can give you a start when they appear out of nowhere, and jellyfish are horrible things to swim into.
It's an uncontrolled environment
Hypothermia is an obvious risk, as are changing sea conditions and the effect of wind, rips and currents.
Sea swimming is almost always colder than pool swimming and this will slow you down, so don't overestimate the distance and time you can manage.