You're not really that far from either Faneuil Hall/Quincy Market or North Station there. Can't speak to how easy it is to find one in any of the local hotels. Of course, in the latter case it helps to look the part and not look too clueless.

> Of course, in the latter case it helps to look the part and not look too clueless.

This is not just applicable to using the restroom as a non-paying customer. You can pretty much go anywhere if you just don't look guilty of doing something your not meant to be doing. They also say carrying a clipboard and looking in a hurry increases your "getting away" with it, whatever "it" may be.

The basement bathroom in Quincy Market is the best option in most of the city.