True Utility has an impressive line of small key ring tools, including the tiny and waterproof CashStash that is designed to keep reserve cash close by in case of an emergency. After being caught without some cash on a recent outing, I decided something like this would be handy for the boat. I've got in the habit of just carrying my ID and a credit card when out on the water, so this is the perfect way keep a little extra cash close by.
Besides being small and waterproof, the best feature is the special cash clip inside that lets you wrap up a bill tightly. It hardly takes up any room on your key ring, yet it's there when you need it.
Constructed from dark chrome plated aircraft grade aluminum with a waterproof neoprene `O' ring, it earns high marks for durability and design. Plus at $7.99 you'll still have some money left over to stash in it, which is nice.
I keep mine on the key ring with my boat key so I'll never be caught without cash again.