NOAA
Noaa: Our Restless Tides
This site provides an excellent explanation of the nature of tides and how they are formed. The one shortcoming is the use of the fictional "centrifugal force", but this is commonly misused in literature.
Other
Quoddy Loop: Tides
This resource gives an in-depth description of the Quoddy Loop of Islands within the Bay of Fundy. This area has the greatest tidal range in the world. Learn what causes these massive tides.
NASA
Science at Nasa: Ocean Tides Lost and Found
This article explains what forces are at work on changing the tides of our oceans.
NOAA
Noaa: National Data Buoy Center: What Are Tides?
This site from the National Data Buoy Center presents a brief description of the nature of ocean tides.
Museum of Science
Museum of Science, Boston: Ebbs and Flows of the Sea
A simple one-page explanation of oceanic tides. Includes instructions for completing a tide-mapping activity.