Watertown is a very good market right now, It is desirable for buyers and renters.
I'm a Watertown resident and an agent in Watertown. I can say that it's hard to find apartments for rent. So you will not have hard time to find tenants. Your investment could be good if the numbers work, that's a calculation that you need to do for each individual property. Some property have a rental history, so you can see the numbers that will come in. The best return is on a tow family home.
I will be more then happy to hear more about your home search.