Tides come in and go out twice daily — and times differ from day to day and from town to town. At low tide, the sandy shore is hard and easier to walk on; at high tide, what little sand is visible is more difficult to walk on. Tides vary considerably from one spot to another.
Tide Charts. (This defaults to Barnstable Harbor, so simply choose your closest location from their drop down menu.)
There is also info on currents, moon phases, surf forecasts, and sunrise/sunset times.