The lack of overt, audible skepticism is a decidedly good sign. Whether or not his explanation is believable really isn't up to him, of course.

"No. Well, yes. The water won't hurt you, but the effects last for about an hour. You might have better luck with teas and juice, although I've really got no idea how much water has to be present for it to be affected." Now's as good a time as any to point out, "For instance... you told me a moment ago that you're a secret agent. Ah. Resigned agent, I mean."