Because the Earth rotates through two tidal bulges every lunar day, coastal areas experience two high and two low tides every twenty-four hours and fifty minutes. High tides occur twelve hours and twenty-five minutes apart. It takes six hours and 12.5 minutes for the water at the shore to go from high to low, or from low to high.
It's (very roughly) 12 hours from one high tide to the next, and of course the same for low tides. That's because the tides produced when the Moon is directly overhead or in the opposite direction are the same. Same with two low tides, since there are two tides a day (both low and high).
