I would be happy to assist you with your search further. Please contact me at 781-330-0152
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.
Watertown is a great area. Nice proximity to Cambridg, Somerville and downtown Boston.
Here is every property for sale in Watertown: http://goo.gl/qHj3p
Unfortunately there are too many variables here to give you a straight answer. This can be dependent on size of the home, price, location, and average rent prices in the area. My advice would be to find yourself a good buyers agent who can help you determine this for each listing you are looking at.