The ocean has tides, which are periods of high water twice a day alternating with periods of low water twice a day. Each full tide (high + low tide) takes about 12 hours. So, at night time, actually, both high and low tide occur. It's likely that during the times that you've observed the ocean at night, it has been high tide, but it's not always the case. The tide is caused by the gravitational pull on the oceans from the moon and the sun.