The federal government’s National Oceanic and Atmospheric Administration has an online site that provides data on tides and water temperatures: http://tidesonline.noaa.gov/geographic.html. So, for DC’s Potomac River the link is http://tidesonline.noaa.gov/plotcomp.shtml?station_info=8594900++Washington,+DC#.
The mobile-friendly site is at: http://m.waterdata.usgs.gov/.
And the online consensus seems to be that fishing is worst at both low tide and high tide. E.g., “A slack tide means that the tide is neither incoming nor outgoing and that the water is not moving much. Avoid fishing during times of slack tide and focus on time periods where tidal movement will be most pronounced.”