Newbie - unintentional First Deal - First issue - separate water?

[Ceril S.]

I've been listening to BiggerPockets and reading info here for a couple months. I've been scoping out different neighborhoods within a certain radius from my house and watching the MLS listings - running theoretical numbers on properties to practice using different calcs to figure out what viable looks like. I even convinced my wife to look at a couple open houses. About a month ago she got a call from her dad saying that he's ready to move closer to us. My wife picks up one of my info sheets on a duplex and says "what about this one, for my dad?" So - fast forward a very quick month, it looks like we will be closing on our first deal early next year.

I know, I know - everyone says "NEVER FAMILY!!"  It seems actually to be a good compromise for us - I get in on my first real estate property and she gets a safe place for her dad that is subsidized by renting out the other unit.  AND we don't end up with her dad living with us! :)  

So here's my first major issue - The water is not separated by unit and there is only one water heater.  How do I divide this between the two units.  I thought about submetering but the piping is not set up to be two separate units.  What are my options? (other than including water in the rent)