Tides are periodical fluctuations of the sea, which occur twice a day. When water is high, it is termed as high tide while in case the water is low, it is termed as low tide.
Tides are very strong on the new moon and full moon days because hey are caused by the attraction of the moon.
This effect of the moon is interfered with the effects of the Earth’s gravity and the centrifugal force produced by the speed of rotation of the Earth, thus, tides are affected by these forces.
Below Tides info from Wikipedia (Read More):-
Tides are the rising of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides change completely every 9 hours.[1][2][3] Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation (see Navigation). The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides (see Intertidal ecology).
http://en.wikipedia.org/wiki/Tides
