1) I've read that your heart rate is lower in the pool. Why is this?
2) Also how does the lower heart rate impact your cardiovascular health? Is it good or bad to that the pool lowers your heart rate?
Heart rates can trend lower in a pool, because your body is horizontal and your heart is not fighting gravity to get blood back up from the extremities. It's along the same principle as your heart rate is lower when lying down than when standing.
As far as the cardiovascular, a high heart rate does not automatically increase your cardio fitness. It's function of the work you are doing, not how fast your heart rate gets. If anything, it's possibly that swimming (in general) is better for cardio, as one of the biggest limiters in cardiovascular exercise is being able to get rid of heat. (Aside from the benefits of non impact exercise).
This would be because when your training outdoors or in gym, your heart rate is faster due to your body temperature being so raised, where as when you swim the water keeps your body temp regulated.
It is neither good nor bad, it is just what happens. Swimming is an extremely good and beneficial exercise.