This one probably seems like a no-brainer to most of you. However, I can't tell you how many horror stories I've heard from people dropping their keys in the drink. One of the cheapest and wisest investments you can make is to buy a flotation device for your keychain.
I tend to lock my truck keys up first thing, so I don't see a need for one there. However, having a flotation device is necessary for my boat keys. I've never dropped them in the lake, but I have had a few close calls. Keep in mind that you'll want a floating device that will actually keep your keys above water in the event you drop them there.
In my opinion, the bigger the floater, the better. If you feel the need, you can do a trial run by dropping your keys in your sink at home. If they float, you're golden. If not, it's time for a new keychain.