A zero-impact, full-body workout that keeps you cool on a hot day, swimming is the summertime cardio of choice for many women. After a few weeks, you may notice some new shoulder muscles, but don't worry -- you won't turn into a bodybuilder. Swimming can't change your genetics, and you'd have to work much harder to get beefy shoulders than swimming will allow. So reap the aerobic benefits of your laps, and show off your new tone.
The width of your shoulders is defined by your skeleton, which is genetically determined and will not change. Some women naturally have broad shoulders and others are naturally narrow -- those with narrow shoulders create less resistance to forward motion in the water and may be faster swimmers as a result, but swimming will not give them broader shoulders any more than being sedentary will narrow the shoulders of the other group.
Well-developed shoulder muscle can visually widen the appearance of the upper body, but swimming will only do this to a certain extent. Beginners may notice some initial toning in the upper body, but this doesn't last forever. Once you've spent enough time in the pool that you have developed the strength to move your body through the water, actual muscle growth will plateau because the load the muscles are forced to bear doesn't get any heavier.
The sculpted shoulders that you see on Olympic swimmers are the result of a weight-training program done in addition to pool time. The stronger you are, the more efficiently you can move through the water because you pull more water with each stroke. Women shouldn't worry about turning into hulking monsters from lifting weights -- building enormous muscles requires not only a dedicated training program, but also a high level of testosterone that most women simply don't have.
Unless you are a very competitive swimmer, muscle development should not be a major worry for you. If you are only swimming recreationally or for exercise, focus on the aerobic aspect of swimming and work on flexibility exercises that can help you move efficiently and maintain proper form. If you decide to join a team, however, understand that you will not realize your full potential in speed and power unless you make muscle development a part of your training routine. In that case, well-developed shoulders are a badge of pride.