In some jurisdictions (MA probably included but I'm not sure) tap water must be free at restaurants[1] for restaurant patrons who are ordering food[2]. An obligation to provide water to someone who isn't buying anything seems unlikely.

[1] I am not entirely certain whether DD constitutes a "restaurant" for these purposes. Table service might be required.
[2] And some jurisdictions go even further by requiring that tap water be given by default to patrons who didn't even request it.