By far the most important factor, effecting the movement of water across the ocean are tides. Tides are great bulges of water, caused by the gravity of the Moon, and Sun. Attracted by gravity, these bulges move around the Earth’s oceans, causing water levels to rise and fall. Typically water will rise for about six hours, followed by six hours of falling water height.
Tides are more noticable near the top and bottom of the earth. The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada, which have a range of 44.6 ft. Below are time lapse photos of the tidal rise and fall over a period of 12 1/2 hours at Nova Scotia's Bay of Fundy. Notice that the boats sit on the dirt during low tide.
Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes. There is about 12 hours and 25 minutes between the two high tides, so we get two high tides as well as two low tides evey 25 hours.
