I think a waterproof key would be cool and would fit the image of Jeeps pretty well.
But I'm pretty sure they aren't even water resistant as they are...when I pulled the key apart to change the battery, I don't recall seeing any seals to keep water out.
One option might be to simply put one key in the baggie and shove it inside your wetsuit somewhere, and keep the second key inside the vehicle, say in the locked glove box. Even if the key got wet, you could still open the door (the alarm would go off), then use your dry key to shut off the alarm and drive away. Sure it might ruin a key, but then you know you'd have a key that you could take with you whenever you dive.
2012 Wrangler Rubicon 2-door
2009 Aprilia Dorsoduro 750
2008 MV Agusta Brutale 910S