First off, is your water heater raised off the ground 18 inches per code?
Second, do you have two vents in your walls/garage door that are at ground level, again per code? The idea is that gas vapors will hug the ground and travel out the vents before they rise to the level of the pilot light on the water heater.
Do a Google search on wireless smoke/heat detectors. Here's one at Lowes
http://www.lowes.com/lowes/lkn?action=productDetail&productId=84145-65213-0919-9999&lpage=none