Harbor Electric Energy Co., a subsidiary of New England utility Eversource Energy the Massachusetts Water Resources Authority are embroiled in a dispute as to who should pay $20 million to repair a construction job done 25 years ago. The issue being raised is whether an electric cable was laid to the proper depth below the harbor. This has gained urgency, with the approach of a $310-million project to dredge Boston's ship channel to accommodate deep-draught ships.