Legal Question in Landlord & Tenant Law in Massachusetts

Re: Landlord rights

When a lease term expires and a tenant remains in possession as a tenant at will, the law will generally imply that the terms of the lease carry over to the tenancy at will. So unless the parties agreed in writing to alter the arrangement or by their conduct altered the arrangement, I would expect a court to enforce the tenants' continuing obligation to pay water and sewer charges.

Re: Landlord rights

The prior answer is what I would have said. However, there is one thing that concerns me: you have said that the tenants live in an apartment, which implies that there are multiple units in the building. water and sewer costs are almost never separately metered. If your tenants were paying for someone else's water and sewer -- yours, if you live in the same building, or another tenant's -- you may have violated the law. The general rule is that tenants are not obligated to pay for utilities that are shared, or that serve "common areas." My recollection is that there is no "wiggle room," even if the agreement is in writing, but I won't say that I have double-checked this in the statutes or regulations.

Even if you gave a very low rent in consideration of this factor, you may well have been wrong.

If my supposition is correct, you should instead create a new tenancy at will by 30-day notice to quit and offer of new tenancy. Factor in the approximate cost of their water in your offer of a new rent.