I used a Poolguard Model DAPT which is UL Listed to UL 2017 and 85 dB Horn At 10 Feet. I just stuck it up with double stick tape for inspection and then removed it. No kids in our family and none ever come over.

It is in my closet if you want it. I think I paid $75 and I would let it go for $25.

we just bought these flip locks for all our exterior doors, but 2-3 weeks after the pool was filled our kids had already learned how to swim. it's a much cheaper alternative.
http://www.google.com/products/catalog?hl=en&rlz=1G1ACEW_ENUS471&q=flip+lock&gs_sm=12&gs_upl=2402l4040l0l6505l9l7l0l0l0l0l219l999l1.5.1l7l0&um=1&ie=UTF-8&tbm=shop&cid=11644425800203186011&sa=X&ei=w8U7T_qaHKTi2gXm3Y3zBg&ved=0CD4Q8wIwAA#

http://www.acehardwaresuperstore.com/index.php?option=com_virtuemart&Itemid=68&page=shop.product_details&flypage=flypage.tpl&product_id=21424&category_id=1819

cheap ones