NOAA
Noaa: National Ocean Service: Education: Tides and Water Levels
A great resource for explaining and understanding tides. Find out what tides are and why it is so important to monitor them. This is an extensive tutorial followed by a set of review questions. All of the information can be downloaded as...
Woods Hole Oceanographic Institution
Whoi: What Causes Tides?
The Woods Hole Oceanographic Institute provides an in-depth but easy to read article on the role of the moon and the sun in respect to 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.