[JohnnyD]

The tides should be only a few minutes apart. You sure you aren't comparing the high tide at Green Harbor with West end of the Canal?

Green Harbor Boats and Boating In Massachusetts - Massachusetts Marine Trades Association - MMTA
Canal RR Bridge Boats and Boating In Massachusetts - Massachusetts Marine Trades Association - MMTA
Canal East End Boats and Boating In Massachusetts - Massachusetts Marine Trades Association - MMTA