There is a better way, and tides and currents data is included with the charts. Maptech's new U.S. Boating Charts with Tides and Currents on DVD has over 2200 up-to-date NOAA nautical charts and more than 750 river charts from the Army Corp of Engineers. It comes with GPS real-time navigation software and U.S. Tides and Currents data for a two year period.
