Complex sentence: The rule assumes that all tides behave in a regular manner, this is not true of some geographical locations, such as Poole Harbour where there is "double" high water or Weymouth Bay where there is a double low water.
Split into simpler sentences: The rule assumes that all tides behave in a regular manner, this is not true of some geographical locations. This is often true but there are exceptions, such as Poole Harbour where there is "double" high water or Weymouth Bay where there is a double low water.