Tides are the vertical movement of the water resulting from the gravitational attraction of the moon, sun and earth. In theory we should have 2 low and 2 high tides every day but not normally of exactly the same level. The daily difference between tidal heights is due to the changing distance between the earth and the moon. High and low tides occur 50 minutes later than the corresponding tides of the previous day. Tides predictions repeat only in a cycle of roughly 19 years.

For us SCUBA divers, tides are an important consideration when choosing a dive site; not only tides, but also tidal currents. Both low and high tides are important in the selection of a site. A low or high tide can make the entry point difficult or make for a long surface swim. Also, tidal movement can cause visibility changes.
 
Tidal currents are the horizontal flow of the water. It is very important to know not only the speed but the direction in which the water is moving. Another consideration are wind-driven currents, wind that is blowing across and causing the surface water to move.
 
The best time to dive is at slack tide. Slack is a period of no discernible flow. Slack is of interest to divers because not having a current means taking less effort to stay at the dive site. Most people make the mistake of assuming that slack occurs exactly at high or low tides, but in some cases it may be hours before or after the high or low. Slack will most likely present the worse visibility because there will be no water movement to remove silt.
 
One rule to follow that is fairly safe for most locations is to avoid the ebb (the flowing back of the tide as the water returns to the sea). It is stronger and lasts longer than the flood (the rise or flowing in of the tide).

The best and most accurate way to determine tide schedules is by using The National Ocean Service (NOS) http://tidesandcurrents.noaa.gov/index.shtml Tides and Tidal current tables. I also use this website to get my information http://www.dairiki.org/tides/. Bear in mind that this site provides estimations. You should always check for conditions at the site before starting any dive.

There are also many books and some software applications commercially available that are specifically designed to assist divers with tides, so be sure to do a search for some of those as well.