The Davis Instruments Key Buoy is the ultimate key fob for boaters and lakeside loungers! When keys fall in the water, the Davis Instruments Key Buoy's self-inflating key ring opens up, releasing an air tube which inflates automatically. The 14 inch (47 cm) inflated tube extends above the water, allowing you to reach over and pick them up. The Davis Instruments Key Buoy rises to the surface in as fast as 60 seconds. May take longer in deeper water with maximum weight attached. Strong enough for items weighing up to 2.8 ounces (80 grams) such as keys, small tools, deck plate keys. Suitable for single use only, so keep a spare on hand!
I haven't had to use it yet, (and hopefully never will) but it is very lightweight and only marginally larger than your normal key fob. it is cheap insurance considering most modern key are chipped and replacing them at the dealer is several hundred dollars.