This is not to say that swimming does not make you physically fit, it does, only that an out of shape athlete that knows how to swim properly still swims circles around a triathlete who spends an hour every day in the pool doing laps. If you just want to complete the distances for the triathlon then, jump in the pool and swim till you feel tired. This may only be one length of the pool. Do not get frustrated especially if you have just pulled off a sub one thirty half marathon and feel fitter than ever before. Once you get your swim rhythm you will feel great.
Swimming is all about efficiency in the water.
Unlike running or biking where gravity and wind resistance push against you, swimming requires you to continually push through. Opinions vary from one swimming expert to the next, but the great thing is you can figure most of it out for yourself just by jumping in the pool or lake. Pools are useful for keeping track of time and distance, whereas open water swimming builds confidence. Most experts will agree that efficiency is determined by the number of strokes it takes to get from one end of the pool to the other. The fewer the strokes the more efficient you are, generally.