Get a blade cut instead that you can just as a manual blade if that's possible with the A8.
I've done in the past for when kayaking then I just leave the key in the car and lock it with the blade.
Just a thought, it's worked for me in the past but I prefer to do that than risk taking my key out on the water.
Best way is non electronic key like first said so glad you went with it. Last thing you want is to have to test your expensive audi key in the drink.
I don't even bother with my phone when on the water. I'm sure dry bags and stuff are really good and do the job but I'd rather not take the risk when you don't have too.
Car will be fine with the key in the car, just don't leave it on the dash saying Hey look I'm the key for the car lol.