The interaction between exercise and you insulin needs can sometimes be kinda wacky. If you exercise with low levels of insulin, and have not eaten your body will naturally release glucose to fuel your exercise, but it may overdo it resulting in high blood sugars. That is probably what you have observed. On the other side, if you exercise after a meal with insulin on board, you may actually end up readily soaking up those carbs and have the opposite effect and end up hypo. Dr. Sheri Colberg has written extensively on exercise and type 1 diabetes (http://www.shericolberg.com/). Dr. Colberg also has a number of good books that are likely available at your public library. In general, a safe way to approach exercise like swimming would be to consumer 15-30 grams of carbs before exercise and bolus for those carbs. Many will find that they require a smaller bolus during and after exercise, so you should expect to adjust. Most people should find that starting exercise with a bit of carbs in your belly and the right amount of insulin is a good safely way to fuel your energy needs and keep your blood sugar balanced.
Hope that helps.