I am amused by the thought of putting some sort of timer in the bathroom before he arrives. Show it to him during the tour, and say, "Now, we've discovered that the amount of hot water in the tank is enough for an 8 minute show, no more. When the hot water runs out, it goes from comfy to ice-cold in the space of a heartbeat -- trust me, you do NOT want to experience that yourself, so make sure you're out as soon as the timer goes off." Would you consider buying a new shower curtain with strong magnets, or attaching some strong magnets to the one you have now?