Mark, in Ct, where I live and install pools, a pool alarm is required by law to be included for every new pool built (as of the late '90s). We provide them (and will install them if asked to - our obligation, legally, ends with our providing the owner the unit), I'd say that over 50% of owners don't install them nor use them. The one we are using now is the Poolguard (that Iwannapool mentioned), it has not only a VERY peircing unit alarm, but also an indoor receiver that sounds stridantly inside the house (I know of no units that can do more than this (as per simsjim's advice)).
One of the problems with pool alarms is that high winds can set them off, and you may have to lose a little return efficiency to keep them from sounding off every ~ hour. IMO, the ones that attach to the ladder are worse than the deck mounted ones. I believe that infra-red 'beam' deck alarms are available, which take the wind and returns (the pool water) out of the picture, but I can see issues with falling leaves and wind blown debris setting them off.
I congratulate you on wanting to make your pool as safe as possible! (I know that I'm not the only one here who has had to call in a 'floater' (deceased person in the pool)). Do some research and find out which unit will be best for your situation - pools should be nothing but fun, however - there's always the chance... - the only thing you want to kill in your pool is algae and bio-nasties!