Swimming is one of my favorite ways to workout on the weekends. It's far less taxing on your muscles and joints than running, but you're still getting a great (and fun) workout.
If you want to get an even better workout next time you hit the pool, think about using some aquatic accessories like water gloves. These heavy-duty open finger Neoprene Water Gloves ($11.95) add just enough resistance to help increase your heart rate during aquatic exercises. The gloves have a "rubber-like" palm and secure Velcro closure for easy on and off. Unlike many swimming accessories like a kick board and flippers, they are small enough that you can just throw them in your gym bag,
Buy yourself a pair from Power Systems online. I know most gyms supply flippers, kick boards and hand paddles and that heavy chlorine usage at most pools make sharing water accessories less gross than it could be, but it's still nice to have your own stuff.