One of my favorite things about the sport of swimming is that it's a non-impact sport. As such, it carries a much lower injury risk than a heavy-impact sport such as running (except in the melee of a triathlon wave start).
Shoulder injuries resulting from overuse and poor technique are common in swimming, however. To prevent these injuries, make sure your technique is in order. The most common stroke problem that causes shoulder issues (although not the only problem) is crossing over the midline of your body at the beginning of the pull.
The second most effective way to prevent swimming-related shoulder injuries is to avoid overtraining. It's safe to say that many triathletes are competitive and sometimes type-A folks who just want to keep up with their training partners, and they are prone to make the mistake of trying to advance to that next level in training a bit prematurely.
If you are this type, you will come out ahead in the long run if you ramp up your training cautiously and stay healthy, rather than if you aggressively increase your training volume and get injured.
I highly recommend that beginners stick to drills as much as possible early on. In the first several weeks of training, 80 percent to 100 percent of your yardage should consist of drills that help your stroke technique.
Keep this in mind if you are just starting out or if you have taken significant time off from swimming and are coming back. I've been guilty of trying to do too much too soon after I've taken time way from the water—and have paid for it with shoulder tendonitis.
Once you overcome that initial hump and you are doing straight (full stroke) swimming for the majority of your workout, you definitely want to increase distance slowly. The definition of "slowly" will, of course, vary from person to person. Some swimmers can handle faster training increases than others. Listen to your body and you will find the rate that works best for you.
Building Swimming Volume
Here's an example of a three-phase, 12-week beginner's plan for building swimming volume. It is best suited to triathletes preparing for Olympic-distance events.
Weeks 1 to 4: Gradually decrease drills from 80 percent of your yardage to 20 percent. Gradually increase yardage from 1,200 to 2,200 per workout.
Weeks 5 to 10: Keep drills at 20 percent of your yardage. Gradually increase yardage from 2,200 to 3,200 per workout.
Weeks 11 to 12: Taper. Increase drills to about 30 percent of your workout. Decrease yardage to about 2,000 per workout before your race.
Again, each athlete is a little different in terms of the total volume and rate of volume change that work best. The emphasis should be on technique first.
Go at your own pace and don't be tempted to keep up with your training mates in terms of increasing distance or dropping your interval times at the expense of technique. Instead, you can compete with them on runs, or ride, or where it counts: at the race.