Okay so I know this is really dumb.
I tucked the spare car key into my swimming cozzie so I wouldn't drop it and then completely forgot and went swimming. I was in the pool for 15 minutes 
When I got the key open, it didn't look soaking wet so I dried it as best as I could and put it in the airing cupboard overnight.
I'll try it in the morning. What do you think - will it be alright?