Hi E.,
I have a pool. I do not have an alarm. I have physical barriers instead which make me feel a bit more secure than an alarm. As one mom pointed out, by the time you hear and respond to the alarm, it might be too late. My mom has alarms on her doors, not the water, for the same reason. The challenge with them is: do you turn them off when the kids are in bed and you're going in and out? If so, do you remember to turn them back on?
First, I have a fence that blocks off the pool. One mom mentioned using a chair to climb over the fence. Easy solution, move the chairs and anything a child might use to climb over. The gate, which is self closing, has two latches and we lock it with a combination lock since my daughter learned how to open the gate. You can find fence standards for pool safety online and also at your local planning and building department.
Second, the two doors that access the patio have pin-locks at the top of them. There is a whole drilled in the top of my sliders that a "pin" is inserted into. This prevents anyone from opening the door. You can find these at the hardware store near the door latches and hinges.
Thirdly, we teach pool safety and respect and have a zero tolerance policy. We do not run around the pool (you can trip and fall in). Children are not to put anything in the pool or take anything out (the #1 way kids fall into pools). We do not play on the pool deck, ever. Our play structure is out near the pool, so when I open the gate, especially when we're having a play date, I remind all the children about the rules. I make sure my children follow these rules at other people's houses too.
BTW: We are fairly easy going parents. We are incredibly uptight and strict when it comes to the pool. We are looking forward to taking down the fence someday as it will enhance the view. Until then, we want to keep our kids safe. Our kids are 3 and 5. We'll have the fence probably until the day they can both climb over it unaided.
Good luck and happy swimming!