We had a tragedy here in CA. last year or so- mom, dad, twins. Mom put the kids down for a nap. Mom fell asleep. The pool gate was propped open. They found both kids in the water. Guy I work with, two weeks ago, in-laws- pool gate left open- two year old. (Current stats for the U.S. are about ten accidental drownings a day.)
5' gate around the pool, or at least at all the bottlenecks. Secure back yard- the neighbor kids KNOW about your pool. Chimes on all the exterior doors leading to the pool. Push button alarms on the pool gates, with Magna-Latches on all the self-closing gates. (I love our Magna-Latches.) In water alarms get too many false alarms- I never used ours. Clear lines of sight from the house to the pool, if possible. (I considered a CCTV...) If your child is missing, even for a minute, go straight to the pool. No floating pool covers- too easy to lose a kid under one of those bobbing body bags. (Motorized covers, though, are great.) Teach your child, even now, that they NEVER go near the pool without Mommy or Daddy-wrath of God style. (When we moved in to this house, the kids were 3,2,1,&1. The pucker factor was very high.) And, of course, Mommy or Daddy are EYES ON during all swim sessions. No phones, no doorbells, no drinks- if you leave, the kid is in your arms. Know CPR.
Spend whatever you need to spend, or don't do it at all. Guaranteed, your child, or a guest's child, will get out, and will head straight to the pool. And will fall in. Unless you do what it takes to stop them. (Here in San Jose, code requires all the things I mentioned. Should be done regardless.)
Tom