All leasehold rights and interests of private individuals, firms or corporations in or to harbor areas located within the limits of a city or town are declared to be real property for the purpose of assessment for the payment of the cost of local improvements. They may be assessed and reassessed in accordance with the special benefits received, which shall be limited to benefits accruing during the term of the lease, to the property subject to lease immediately abutting upon the improvement and extending one-half block therefrom not exceeding, however, three hundred fifty feet.