Ever wondered why there are low and high tides?
This is because at the point directly under the moon, the water is at its closest to the moon. Therefore, there is more gravity which causes tides to rise higher. The water is at its furthest from the moon at the opposite side of the earth so the tides will not rise as high. A tide is a rise and fall of sea levels caused by the combined effects of the moon, the sun and the rotation of the earth. Most coastal areas experience 2 daily low and high tides.
View this video on different types of tides: http://www.youtube.com/watch?v=kHvBUDk7kkQ&feature=related