FYI...there are very few "dive" hotels in downtown Boston, outskirts, yes, but not right in town. I highly recommend a walk through the Back Bay, the Common and the Public Gardens. I think Back Bay is the nicest area of the city. And you don't want to stay in Cambridge and have to train into downtown everyday. The T sucks, avoid it at all costs.
I'll also echo Mike's comment on the Marriott Long Wharf. A good value right on the water. Probably the cheapest you'll get too.
Here are some others to consider:
Jury's (the bar-restaurant scene is very good)
Wyndham
Courtyard Copley
Radisson
TripAdvisor is also a very good site for finding places to stay and eat.
http://www.tripadvisor.com/Hotels-g60745-Boston_Massachusetts-Hotels.html 
