Interesting story, I accidently washed some DS games that were in my pocket. One of the game's plastic case came apart and the circuit board seperated and washed along with it. I let them dry out for a day, expecting to have ruined games, or at least the inability to save. To my surprise none of the games were even phased. Just water damage to the labels. All of them retained their save data, and appear to be in working order, if the one that the circuit board came out of.
So DS games waterproof? Apparently.