I bet most swimmers or polo players below Olympic level have never given any thought to their heart rate while training, but in sports like cross country heart rate monitors are frequently utilized in high level training. Runners for example use heart rate monitors so they can keep their rate in an "optimal zone". By keeping their heart rates in the optimal zone, they can ensure that their workouts are more effective.
When swimming, it's not normally possible to use a heart rate monitor, but that doesn't mean athletes of the water can't also be benefited by heart rate training. It just takes using more ancient methods to find the heart rate: Count the beats over a 15 second period, multiply by 4, and voila
The optimal heart rate range differs by age, and to find yours, refer to the picture, or use this method:
"To calculate your target heart rate, you will first need to know your maximum heart rate (MHR), which is calculated by subtracting your age from 220. Then multiply your MHR by 0.6 to get your lower exercise range and by 0.8 to get your higher exercise range. Now that you know your range, you can monitor your heart rate to ensure it stays within the low and high values." [link]
Check your heart rate periodically throughout your workout. If it isn't up in your range, you know you need to up the intensity of your workout. The longer you can remain in your range, the more effective your workout is, and the more your endurance will rise.